I already made my investment of 500 € back already (my rig went online a week ago). Not in Bitcoins or hard fiat-FTW!!1!-money but in phun.
For the first time in 8 years I've had the time and incentive to once again build a computer from scratch and have it work right away. Other people pay more in a house of pleasure for this...
Heck, I wasted electricity in 2004 solving the Zeta function for ZetaGrid! This is much more fun!
I don't care if I don't recover the initial investment, I will keep going as long as I make more than to cover electricity (at .22 €/kWh) and possibly even beyond that. Just for the lulz.
If you want to make a quick buck, invest. If you decide to mine, be aware that your costs may be sunk in a couple of months. But for me they will be sunk for good because I'm a geek. That's why I don't listen to AngelusWebDesign's panicking.
Yep, the fun and community covers all my expenses ^_^
I don't care for AngelusWebDesign's systematic propaganda one bit.