I'm not quite as big as Phil.
I priced out a 2-card rig on Newegg this past week - fairly conservative estimate 48MH/s (I don't trust most of the posted MH figures, they seem to be fairly old and bigger DAG files slow cards down) at a little over $1000, would earn ballpark .48ETH/day or $6ish right now on an electric cost of ballpark $1.20/day
I had the ROI figured on more exact "current" figures at 255 days.
I did the same on a 1-card and on a 4-card rig (3 cards takes an EXPEN$IVE motherboard to get proper spacing, or you end up crowding one card on any 3-slot-only board).
The 4-card rig was deliberatly designed using blower-type cards for heat management.
ROI worked out to higher on the 1-card rig despite being able to go VERY cheap on MB/RAM and such (mini-ITC AMD A1 slot stuff, but gold power supplies don't EXIST below about 450 watts so couldn't get REAL cheap there).
4-card rig ROI worked out to be 254 days, but most likely the cards wouldn't be able to be pushed as hard as in the 2-card rig so I suspect the ACTUAL ROI would be a bit longer.
VERY VERY iffy on being able to achieve ROI, as the current price bump isn't looking real stable, hashrate is GUARENTTEED to drop over time as the DAG file keeps growing, I just couldn't justify building a complete rig out of NEW parts for ETH mining.
I can *mabye* justify it on the basis of "mine ETH to recoup some of the cost, the do my other stuff afterwards" if I set it up with NVidia cards instead of the AMD ones I specified - but then it earns less ETH and probably becomes unprofitable sooner, or fails to achieve ROI before ETH goes POS, so I'd have to justify such a rig on the basis of "going to buy it ANYWAY" not on "I can pay for this with reasonable confidence before ETH goes POS/unprofitable".
Still, for the short term for those of us with hardware that CAN mine ETH already on hand (or MOST of the parts already on hand), ETH is certainly a good option over a mythical miner that might not show up for several months IF ever.
I've not looked into undervolting my GPUs yet - PrecisionX won't let me undervolt my NVidia cards (and research indicates I'd need to mod the BIOS on them voiding the warrenttee), and I've not looked hard at undervolting the AMD cards yet (I know the tools exist, just haven't had time to dig up the right ones yet).
Several points:
1. GPUs have good resell value, which may or may not change due to Polaris, etc. You should get at least 50%, maybe more. the rest is reusable anyway.
2. Who says that Polaris will be delayed? AFAIK, it will be introduced on June 1 and go on sale few weeks later, maybe as early as late June.
3. Spacing is not that important since most people use powered USB risers, then hang the cards on wired stands with low cost like zip-ties.
My wife calls them "drying fishes".
Example-I run 6 GPUs on one rig using ASrock 97 anniversary (6 slots one)-just $60 on newegg.
4. True, starting mining eth in January-Feb would have been better than right now.