Wirelessly posted (BlackBerry8530/5.0.0.601 Profile/MIDP-2.1 Configuration/CLDC-1.1 VendorID/417)

I use the Q10, and the slightly faster Q10 Limited. Travel time is about 35-40 minutes between Kew Gardens and Terminal 4, so it will normally be a little slower than the AirTrain, especially if you're not going to/from Terminal 4.
The bus can be crowded when leaving Kew Gardens, but the buses leave very frequently, so if you need to wait for a second bus, you should then be near the front of the boarding line, and won't have to wait too long.

Just be aware that while all Q10 Limiteds go to JFK, only some "regular" Q10's go all the way to JFK. Be sure to check the destination sign before boarding.