I started with the Block Erupters in October.  Since I got 2 for free and 4 for $10 each, ROI on those only took a couple weeks, however I reinvested all that in more, again for $10-15 each.  Once I got 8, I had to get a hub, which took another week and a half.  All together, it will be three months next week and I currently still have 0.11 BTC.

So for me dollar wise with electricity an all, two months ROI and a month to get the 0.11 BTC.  My son bought 10 Block Erupters all at once for $15 each and it took him three months to ROI.  Last week we combined our hashing power into a single worker on BTCGuild and we get a payout per day (24-26 hours) at 8.4GH/s, which at this point is all profit minus our electricity costs.