There are people on here who know far more about this than I do, but on the offchance that they miss the thread, I'll give you my two penn'orth.
Generally speaking the differentiators are fourfold, namely effectiveness (how well they actually shift), reliability (likelihood that they'll go wrong), weight (self explanatory) and strength (ability to withstand sustained abuse). Looking at the normal Shimano range (as opposed to Saint which is downhill specific) Deore offer superb value for money and many people (myself included) run Deore kit on All Mountain bikes designed to do fairly hard core mountain biking. Below Deore level, I'd suggest that the kit (whilst there is nothing worong with it per se) isn't really designed for proper mountain biking and is less likely to retain effectiveness and reliability in difficult conditions. Above Deore you have SLX which many believe is the best compromise of the four criteria listed above. Above that you have XT and XTR, which is expensive and really only justifiable (particularly with XTR) if you're a racer or have more money than sense.
So, the answer is that it depends on what you want to use your bike for. If you're going to take it to trail centres and onto muntains and throw it about a bit, then get the Deore, no question. Easily worth the few extra quid.