Korg nanoKEY - $50 (5.5/10)
You have to keep in mind that this score takes price into account. This little guy is total shit compared to a normal full midi keyboard. It goes great with a laptop though, having a similar feel to a computer keyboard, but it only has two octaves limiting its use as an actual playing keyboard. I've found that it's surprisingly good for triggering drum samples. I bought two of these, a white and a black. The white one broke after about a month of playing, but the black one still works well today.
-That fucking CC Mode button is too close to the keys
-Doesn't play much like a normal keyboard
-One seemed to break easily
M-Audio KeyRig 49 - $100 (6.5/10)
The KeyRig (any of them in the series) is worth well more than two nanoKEYs at twice the price. The keys don't feel much like a piano, but that's fine if you're mainly using it for synth. My main problem with this keyboard is the shitty USB plug in the back that got fucked up after a few months causing me to have to hold it at a certain angle with duct tape. Then it stopped working altogether. It was pretty good while it lasted.
-Decent keyboard feel
-Plenty of keys
-That fucking god awful USB plug
Behringer U-Control UMX610 - $170 (8/10)
The UMX610 really does a beautiful job at being a USB/MIDI keyboard. It has a great feel, with realistically weighted keys, and plenty of knobs to assign to whatever you want. It also has a professional looking design, which is hard to find in a fairly cheap USB keyboard. If you have the money, I'd definitely recommend this. Behringer knows what they're doing with this one.
-Plenty of octaves
-Lots of knobs
-Similar plug problems to the KeyRig 49, but to a lesser extent
-There are fucking dog hairs all over my keys