I too was like you when i first got my n900, but impatience got the better of me but ive not had any problems insyalling from the testing/devel repo's other than using up my rootfs (which seems to be totaly fixed sincr pr1.2) and ive been using them for about 4 months.

My best advice would be to learn to flash your n900 and do regular backups of anything you dont want to loose, that way whatever happens the worst case senario would be a reflash and restore.

You cant do any permanent harm using these repo's other than loosing data, as log as your prepared to reflash you'll be fine, and its something thats well worth learning anyway.