I'm currently in the process of building my own 'sous vide' cooker using this guide as a starting point (though I'm going to enclose my tank and provide some insulation). Assuming the temperature is accurate and stable (easy enough to check with a good thermometer), there's no reason why a diy solution isn't usable. Of course, the downside is that the diy solution is ugly and unwieldy ... but hopefully it will serve its purpose of allowing me to try sous vide without much expense.