As a long time user of mechanical keys for gaming, I'd suggest getting reds; I personally like the very light taps needed for reds, although it can take some getting used to. If you tend to rest your hands quite heavily on the keyboard blacks are better. For typing they're more or less fine for general use, but as prior comments have stated blue is better for productivity.
Regarding the K90: The wrist rest is nicely angled and firm, I tend to wrest the bottom of my palm against the upper area, having my wrist follow the angle down, and my arm on an arm-rest, it's nice and comfortable. It's not very cushioning; the surface has a rubbery smoothness to it, which makes it a comfortable rest (along with the small circular indents, which prevent your palms getting sweaty), overall it's a good wrist support, but if you want something comfy (more akin to the wrist rests you had in the old mouse arenas) this is not the wrist rest for you. Personally I like it.
Build quality of the K90 is sturdy, no problems so far, and I've had mine since release. Backlighting is a bit uneven, the main keyboard is lit clearly, however the Fkeys and Home block are somewhat dark. The numpad and macro keys are nicely lit. Media keys and volume scroll thing are nice. The macro record functionality, and general software functionality is reasonable - it's not the most intuitive macro software, but it's not nearly as bad as some have claimed (ie. people tend to rant about it rather than actually finding out how to use it). Regarding the design, personally I like it, the brushed aluminum nicely diffuses the light from the LEDs, as the keys aren't actually built into the body of the board, but rather resting on top of the aluminium, this is visually pleasing. This design also makes it quite easy to clean if you take the key caps off. The only downside is that peripheral keys aren't mechanical. This doesn't cause any issues to me, as I tend to use mouse buttons instead of F-keys, but for those who are frequently using F-keys or macro keys when gaming, this difference in touch might get annoying. I believe corsair stated that it was intentional such that the user could easily tell when they hit F-keys, frankly I think that's nonsense, but it's not entirely untrue, it's somewhat useful when getting used to the position of the macro keys, as you know you've hit one - overall it'd be nicer if they were mechanical though.
Overall at their current price I think they're a good buy, back at the original price of £120, I felt they were pushing their luck, but at £90 they offer suitable quality for the price.
[Edit]: I just read the post above me, regarding the difficulty of learning to use reds - I would have to agree to an extent. Very soft taps can trigger the key, and while this can be excellent for double-tapping, or 'nudging' in games, you will probably make a lot of mis-hits for the first couple of weeks
.Edited by Wampbit - 11/11/12 at 3:46pm