It comes down to preference, but I very much prefer using a single Joy-Con sideways (e.g. Mario Kart 4 player) with the strap than without. Buttons feel mushy compared to without the strap, but they're much larger and it makes the Joy-Con nicer to hold. It'll also be useful for motion controller games such as Arms.
There a couple significant things to point out:
-1) The Colors are not the same as the Joy-Cons themselves. They are intentionally designed with a slightly different hue for complimenting the Joy-Con color. But quite frankly, I think the Gray, Blue, and Red Joy-Con are all ugly. I'm personally trading in my Gray straps for Black straps, which look much nicer and work with any color Joy-Con. I haven't seen the yellow, so no judgement for or against there.
-2) YOU DO NOT HAVE TO PUSH THE RELEASE BUTTON ON THE JOY-CON. Many people complain about how difficult the Joy-Cons are to remove, both when put on correctly or incorrectly. However, these people are trying to press the Joy-Con release button on the back of the Joy-Con - that is only for the Nintendo Switch itself (I haven't check the Joy-Con grip). The Joy-Cons are designed to be release without bushing this button. The only locking mechanism is the grey tab at the bottom. As long as the grey mechanism is unlocked at the bottom, just put some force into it (no, it won't break on the slider) and it comes off nice and smoothly - put on correctly or upside down.
Do I recommend the Joy-Con straps: Yes, absolutely.
Do I recommend the colors: No, not at all. Go for black, maybe yellow, if you care at all about aesthetics.