I've read the first 12 volumes so far. Got a bit bogged down in his interpretation of Romans 7, but tremendously useful apart from that - if you can speed read, that is (I can't - it's taken me 4 years to get this far!)
Haldane, Murray, Moo and Hodge (in no particular order) are all worth getting, if you can afford it.
Edit: Murray is now only done in paperback, I think. It's still a hardback price, though.