I did the $50 bonus for opening an account (any $ amount) and will be able to close it and keep the $50 on August 6th. Unfortunately, the stock I picked was ARR, a REIT, and between the stock tumbling, and the $6.95 per trade, I'll only have made $20-25. Not bad for clicking a few buttons though.
I would never consider putting more than the $20 I used for my bonus just for the sake of an account bonus. If you are serious about investing in no load mutual funds, do what everyone else here does and use Vanguard. Sharebuilder is probably ok if you must own individual stocks.
It was worth it, but I'd keep it to yourself, other people will probably laugh at you unless they are similarly minded. I'd also pick a low risk stock...coca cola maybe?