967 uses 20x20mm and 20x40mm Bosch extrusion, and like 341 we often tap the ends for 1/4-20 for making corner connections. We like the 5 mm block nuts and small corner gussets. We sometimes use 10-32 screws in the M5 block nuts, because they fit nicely, and we have lots of 10-32 hardware. For the last three years, we've used a kit frame for the drive with Bosch structure coming up from there. In 2009 we also made the drive base out of Bosch, but that consumes a decent amount of precious time compared to tossing a kit frame together.
The sliding block nuts are pretty nice sometimes. For example, last year our shooter used a pair of 20x20 extrusion rails as fixed ball guides. Before we built it and played around with it, we didn't know the exact angle or distance from the wheel that we'd want, so it was nice to work with an easily adjustable mechanism.
http://www.flickr.com/photos/lmrobot...in/photostream
T slots and block nuts can only handle so much force, and if you exceed that the T slot edges bend outward, allowing the nut to come free. So when something is going to get loaded up, you have to drill it out and bolt it through the extrusion.
We used to build frames out of 30x30mm and 30x60mm extrusion, but we switched away from that because it's heavy. 20x20 mm is a nice FRC size.
It would be cool to prototype with Bosch and then transfer the dimensions we come up with to a lighter, tighter sheet aluminum design, our team has a ways to go before we'll be able to manage that sort of design cycle within a 6 week build season.