I've used plenty of stuff in devel and testing, and have never had to re-flash my device, not even once. (I did it once, a day after I got it, just to make sure I had the setup right and knew how to, but I hadn't even setup the repositories at that point...)

If you're cautious about what you install, and disable it after you get what you want, you'll be pretty safe. Just understand, anything in devel may bust things enough to need a re-flash. Sometimes things may even run fine for a while and suddenly start causing problems (like the Pecan bug recently that only started happening when DST kicked in. )

As for bricking, it's pretty impossible on the N900 from a software perspective if you have a home PC. You can always hook it up to the PC and re-flash the software, since there's a non-erasable copy of the flashing code in the ROM of the device. The only thing that can kill you is hardware failure, like a bad USB port or frying your CPU or some other component due to severe over-clocking. (Which has yet to happen, btw.)

So, make a backup, grab a stable image and flasher tool for your PC and give it a shot. Worst case your device goes 100% south and you re-flash it back to a known state, then restore from backup. Just don't do it when you're on vacation, expecting a call for an interview, or don't have a hour or two to re-flash it IF it breaks.