The other big thing you have to consider is resale value. You can plug numbers into ROI calculators all day, but bitcoin mining is so unpredictable that you never know what is going to happen. Always having a viable out with hardware value to offset you expenses is a huge bonus. I'm currently sitting on probably $15k in hardware at resale value. It'll be relatively easy to move as it has a diverse market. Selling 100 usb powered ASICs in an uncertain market, to a very small niche, 6 months from now, is an abolute crapshoot.
+1
Everything will have a better resale value compared to asics, you can resell GPUs, FPGAs, etc. FPGA's might actually hold value better than anything else.
In my opinion, unless you are fixated ONLY on mining BTC or other SHA256 coins, an FPGA would be toward the bottom of the list as to retention of value, based on what is CURRENTLY the state of mining algorithms. It will certainly be displaced by an ASIC of any type for hash/energy/$, especially moreso for ASIC devices on a scale size (A BlockEruptor is fine, but you have to pay each time for the casing, the board, heatsink, , the data connector... a Jalapeno, you get more ASIC chips in a different footprint, but pay once for the case rather than about 18 of the same block eruptor cases... a Asicminer blade, continue along, you're paying for one case rather than dozens of jalapenos, etc.)
At the moment, for a purely widescale market of the entire world of miners and mundanes alike, a GPGPU (video card) has a better resale over its first couple years of life than any device. I could sell my brother my Radeon 7950 in one-two years for 50-75 percent the price, and he could use it. Say I had a ZTEX Spartan6 (bitcoinfpga.com) bought in Feb 2012, $414, 190 MH/s, 8.5W, and decided to sell it for about 75 percent... 300 bucks is about same as a BlockErupter.
So, you have the choice between it, a BlockEruptor, or a Radeon 7950, same price. One produced 190MH/s at 8.5W, One produces 300 MH/S at 2.5W. one produces 550 MH/S at 200W. You asked me "Can I use it for anything other than SHA256 mining?" For all practical answers, for options other than the Radeon 7950, the answer is no. For most of the world, miners and mundanes alike, we have no use for an FPGA as-is if you rule out mining. The one item that has the resell value is the video card... and to a miner, there is no way they would choose the $300 190 MH/S 8.5W device over the readily available $300 300 MH/S 2.5W device, especially given the ability to put more in a smaller physical footprint.
I tried asking the question on here, trying to creatively think, of if you could retrofit an FPGA with memory to make it viable for Scrypt mining, giving it extra life... and was given the BFL Josh "quarter buy a clue" video link. At the moment, with the open availability of BlockEruptors... there just isn't any reason to buy an FPGA at a loss against the BlockEruptor pricing. (I'm not even starting to list pricing of the Avalon chip buys, the Avalon batches, and the BFL preorders, because those for all intents and purposes aren't available for immediate mining RIGHT NOW to the general public.)
Now, if someone comes up with a third algorithm... something that isn't memory intensive like Scrypt, but isn't SHA-256 based that ASICs can't compete in... then FPGAs will end up having a better resell value and will be on top of the heap until an ASIC can be developed and brought to market for that (which is tending to be about 12-18 months turnaround, it seems.)