I've also been toying with the idea of upscaling for a while now. I recently moved from an area where I didn't know many people to one where I've got some thirsty friends, so 5 gals can go pretty quick. The biggest thing for me is that my current setup requires me to do some manual lifting to transfer liquids. I can see that being a backbreaker with 10 gal batches, so a pump is definitely in order. And at that point, you may as well build a HERMS or RIMS, right? So it's basically been my cheapness holding me back.
However, I think I might try "high gravity brewing" for my next batch. Basically, I'm going to tweak the recipe to collect enough wort to boil down to 7 gals, split between 2 fermenters, and top off with some pre-boiled water to get back up to 10. If that goes well, I may just stick with that system for my more popular brews for session drinking with the buds and keep doing 5 gal batches of higher grav beers for more "contemplative" evenings.