I think the key for mining is making back your investment as fast as possible while the BTC is still trading at a fair price and the difficultly isn't too high. Once you make your hardware cost back, everything past that (assuming you aren't paying a ton for power) is just profit. And even if the market did crash and bitcoin had an epic fail, you'd still be sitting on a ton of used gear which you'd be able to sell for even more profit.