It's recommended for glass tanks here, and as mentioned before, there are tank manufacturers that won't warranty the tank if you don't use it. I always use it, it's about the cheapest insurance you can get IMO.
I have had 1 problem with it tho - once I sat a 55 gallon tank on its stand and didn't realise that as I put the tank down, the styro moved about an inch to the left, which left the end of the tank overhanging the styro by about an inch. I didn't notice till after the tank was filled and all back together. About 3 weeks later there was a loud bang, and water started leaking from the tank. I had 2 Oscars in there, so it was a mad rush trying to get them out, collect the leaking water, turn off electrics, etc. When the tank was drained I checked it, and sure enough, the crack was through the bottom, directly over where the edge of the styro had been.