There are two ways to review this TV. The first is relative to its price and the second is independently of price. For its price this is a 5 star TV, but compared to the alternatives it's a 4 star TV, maybe 3.5 stars depending on where you set your standards.
For me I'm comparing directly to a Sony 900E. Widely regarded as the best LCD TV of 2017, the 900E is almost an unfair comparison for the TCL. But if you're here trying to make an educated decision and like me trying to decide whether the extra money is worth it, then this is the review you want to read.
I purchased the Sony 55" 900E on Black Friday for $999.
I purchased the TCL 55" S405 one month later for $399.
So what does an extra $600 get you? Much blacker blacks, whiter whites, more vivid colors, a wider range of colors across a gradient, less bleeding and blooming, and far better uniformity across the screen on single color images.
First, allow me to say I calibrated both TVs to the exact specs recommended by Rtings.com, put them side by side, and played the exact same Youtube 4k Ultra HD videos on them simultaneously while comparing any differences I could find.
Blacker Blacks: Exactly as it sounds, when a black background was on the screen, with a small logo in the center of the screen like GoPro, the 900E has blatantly blacker blacks. The TCL can only get down to a dark grey. This will be most noticeable on the screens that are dominated by a single dark color in the background. In movies that have a lot of dark scenes, this will also come into play, and in a side by side test, will look pretty bad. This is the most noticeable deficit the TCL has to the Sony, and will be visible with the naked eye, even to casual viewers.
Whiter whites: Again, exactly the way it sounds, when the 4k video shows you a snow covered mountain range, you can tell the Sony's whites are a bright white and very clean. The whites on the TCL have a yellowish hint or hue to them, but this isn't nearly as noticeable as the blacks mentioned above. Without this TV side by side with another to compare, you might never notice this, and honestly you can alter the 'temperature' of the TV from 'Warm' to 'Cool' to get whiter whites, but you usually sacrifice skin tones when doing this it seems.
More Vivid Colors: The Sony may be cheating here, as when I put these TVs side by side, I could argue the Sony over-saturates its colors. The TCL's reds and greens seem faded comparatively. Honestly, I actually think there could be a happy medium between these two, with more pop than the TCL, but less saturation than the Sony. In fact, if someone told me they felt the TCL was more 'accurate' to real life nature scenes, I wouldn't disagree. Altering the Red/Green Tint balance on the TCL forces you to sacrifice one for the other, unable to make both more vivid. Adjusting the 'Color' slider can definitely improve the greens, but it starts making reds look pinkish the higher you go, and you lose some depth.
Wider color gradients: I didn't really notice this until a sunset or sunrise video came onto the screen, so consider this minimal. But the Sony was much better at showing you a more colorful gradient transition when a sunset faded from purple, to red, to orange, to yellow, etc. Whereas the TCL felt like it just didn't have the same range of color, it would often appear it was just missing some colors. On a sunrise the TCL would almost appear as a full white light, versus the Sony where you could see the white center, yellow outer, and so on.
Blooming: Because the TCL can't get as dark as the Sony, it means that when there's a bright spot on the screen, it effects more area around that specific spot. Think of a Moon in the night sky, the Sony will show you a crisp moon on a very dark background. The TCL will show you the moon, with some lighting around it, because it simply can't reach the same black levels as the Sony, so the immediate area around the moon will look dark grey instead of black.
Bleeding: The Sony is Full Array and the TCL is Edge Lit, so this isn't really fair, but if you want to know what you're paying for with the Sony, or sacrificing with the TCL, this is the technical aspect. An Edge lit TV gets it's light source from the Edge of the TV, this means that you'll see what appears like bright spots, or splotchy-ness on the TV. Most noticeable around the edges of the TV where the light source is coming from. This goes hand in hand with the next topic...
Uniformity: Because the light comes from the edges of the TCL rather than from directly behind the TV like the Sony, it means it's incredibly difficult for the screen to look consistent across the TV. This is really only an issue when displaying screens that are the same color for a vast majority of the screen. Consider when Netflix is loading, that Red Screen, with just the Netflix logo in the center. On the Sony that Red Background is the same shade of Red no matter where you look on the screen. However on the TCL you can definitely see that the TV is unable to produce the same color consistently from edge to center. This doesn't really matter for a lot of regular TV or movie watching though, as those type of screens are rare, or at least only displayed momentarily when transitioning between scenes most likely.
It's without question that the Sony 900E is a better quality TV across the board. So why 4 stars for the TCL? Because the TCL is $600 cheaper than the Sony, and if you never put them side by side for direct comparison, the TCL is a great TV independent of price, and an amazing TV for the price. The only real concern listed that will noticeably impact your watching experience is the black issues in my opinion. There's a reason OLED TV's are the cream of the crop right now, and it's because they have the blackest blacks in the game. In a lot of movies, there are so many dark scenes these days, and the clarity of these scenes struggles when your blacks are the same colors are the dark grays that the movie is trying to display.
In my personal opinion I would not want the TCL to be my main TV for my living room or family room, which is where I watch all of my movies. BUT as a spare or guest room TV, a TV for your kids, or your bedroom, or in my case a second TV specifically for gaming, the TCL is the smarter choice. We, or at least I, often get caught up in wanting the best of the best, the latest and greatest, and when you're at the store you see everything side by side so you can easily spot the differences. Once you get your TV home, there is no longer a comparison point. Your eyes will adapt to what you buy, and you'll never remember the colors of the TV that was next to it at the store.
For the vast majority of people looking for a good budget TV, I can easily recommend the TCL S405 for you. If you're a pixel peeper, heavy movie watcher, or home theater enthusiast, then you'll likely want to spend a bit more and invest in a higher quality set. The Sony 900E has noticeable upgrades over the TCL, but those upgrades won't be worth it to everyone, and a few of them won't even be noticeable if you never set it next to a TV that costs twice as much. If you set the 900E next to an LG B7, some of the same concerns could potentially arise, but it's another $500 jump in price. You get what you pay for. The TCL is worth every penny, but so is the Sony 900E. If price is your largest deciding factor, I wouldn't hesitate to purchase the TCL. If picture quality is your largest deciding factor, the TCL can't quite keep up with the high end TVs currently available.