You cant help but be impressed by Melk Abbey. It dominates the town and is very striking. The guided tours are a couple of EUR more than the normal entrance price but are hardly worth it as they spend the first half hour showing you round a rather boring exhibition, you'd be better getting the audio guide and doing the visit at yur own speed spendiong more time on the bits you want to see. Definitely worth visiting but to be honest the outside was almost as interesting and impressive as in the inside..
I think visiting St Florian Abbey is probably better