I've been thinking of one myself and have many friends into SM...not S&M.
The DRZ is a solid platform but it will never be a race bike or comparable to a CRF450. But the whole point behind it is that is it ready to go from the dealer and will make a great "street" bike. For the price it's a deal as well. The basic SM conversion starts at about $3500 for wheels/tires/modified forks/brakes/and fender.
The two issues i have with the current DRZ/SM is the 140 rear tire and the fact that the new RM's use a 450cc motor and it stands to reason the SM will soon have the engine.