I purchased the TomTom Spark because I was excited at the prospect of only needing one device (two if you count an optional heart rate monitor strap, sold separately) when I work out instead of also needing to bring my phone or iPod along with me to supply music. This device accomplishes that, but ultimately, I ended up returning it because it lacks other functionality I need.
I primarily use a heart rate monitor for metabolic training, and I need to be able to setup intervals within specific heart rate zones, i.e. a 3 minute interval in zone 1 and then a 2 minute interval in zone 2, repeating. This particular model doesn't come with the optical heart rate monitor built in, but it does work with an external Bluetooth heart rate monitor strap.
While the TomTom Spark has an interval training mode and a zone training mode, they function completely independently of one another rather than together. You can either use the interval timer to do a 3 minute interval and then a 2 minute interval, repeating, without regards to heart rate zone, or you can zone train in one of five heart rate zones but not alternate between multiple zones based on time or distance. The interval timer also only allows for 2 timers that cycle and repeat with a warmup before and cool down after. It does not allow for fully customizeable intervals, i.e. two 2 minute intervals and then two 3 minute intervals, repeating).
If TomTom releases a software update to integrate the interval and zone training modes, I'd most definitely repurchase this item.
Aside from lacking the specific advanced functionality I was looking for, I think this is a pretty good product. Despite other reviews saying this watch is big, compared to other comparable products out on the market, the TomTom Spark is actually relatively small and light. I was pleasantly surprised by this given that it is the only heart rate monitor/GPS watch on the market (as far as I know) that can also store 3GBs of music. It also pairs nicely with other devices. I was able to connect my Jaybird Blueblood X headphones, heart rate monitor strap, and phone via Bluetooth with ease. It was able to find all of my devices quickly.
I agree with other reviews that said this device is difficult to setup and that the interface isn't intuitive to navigate through. However, as someone who has experience with these types of devices, this problem is pretty much universal across brands and models of comparable devices and not specific to the TomTom Spark. My other complaint is that the TomTom desktop software and mobile app used to update this device are not one-to-one experiences. The desktop experience has much more functionality than the mobile app, and in general, the mobile app could use some work.
If you're looking for and intermediate fitness device that is more than just an activity tracker but less than an advanced marathon or triathlon training device, this will probably work for you.