I ate at BLT Burger a couple times this year during a stay at the Mirage.  There was never a wait and the atmosphere is a fun and easy.  I had the Tex-Mex Burger and the Shaft's Blue.  I also got a side of onion rings once and waffle fries the other time I was there.  Overall the food was fresh, cooked to order and tasty.  I did think it was a bit too expensive for what it was, but it wasn't totally out of line with Vegas prices.  I still like Burger Bar at Mandalay better, but if you in the middle of the strip and want a darn good burger I wouldn't hesitate to hit up BLT B.