I love my V3, having used it for nearly one year and several thousand miles in all conditions. Despite the usual English weather and being quite often in big groups (clubruns and races), I haven't seen a single glitch in the data and the V3 never let me down. The display is big and clear (but the CS200 seems good as well from the pics I have seen), the lap functions are useful to compare several laps/segments after the ride (average speed, HR... for each lap). The fact that you only have one sensor for both cadence and speed is nice as well (one less battery to change!). On the other hand, you can only use one heart zone at the same time (you can configure several but only use one at a given time) and I don't find the feature very pratical to use, so I do without.
As I'm becoming a bit more serious about my training, I now need the ability to upload to a computer and especially want my computer to guide me in my exercises (intervals...) so I will probably move to Polar (CS400 or 600) soon. But will definitively keep the V3 for the 2nd bike!
The V3 also uses a 2.4GHz digital transmission which is supposed to be more reliable, the CS200 (and 400) apparently uses an older technology, hence the stories about interferences we can find on forums.
If you don't need to upload data and can find at a good price, I would suggest the V3!