I've had remarkably less than optimal experience using USB wifi dongles with rt3572 and rt2870 chipsets (IIRC) with Ubuntu kernels 2.6.28 thru 2.6.38 or so, even with the modules downloaded from Ralink directly. Inconsistent thruput, inability to change channels, and random disassociation from the AP. Plus, their modules vomit lots of pointless debug info to syslog, with no options to disable it.
I guess I should have mentioned that. Ralink is a starting point, not a finished product. Everyone manufacturer who uses them messes with the driver in their own way. The generics are the worst.
Simply using the built-in drivers in ubuntu or a prebuilt module from ralink yields an almost unusable link.
You have to be on your toes and willing to compile your own drivers (and even learn about the source and make tweaks) to really get ralinks to sing. The good news is that ralink makes this source freely available.
Atheros's work great but they cost more (like $15 or $20 instead of $7) and you still have to be careful to get the right "mad-wifi" drivers. There are less of them though and most versions work out of the box at least acceptably.
If you're preparing your robot army for world domination on a budget, its probably worth putting in the time to figure out a ralink. I've had great luck with this sub $7 beauty http://www.amazon.com/150mbps-Wireless-Adapter-Wifi-802-11b/... but it did take considerable effort to hack the driver into shape.
I'd recommend Atheros arl9170 instead.