Pros: Great sound and comfort, AKG service
Cons: A little hard to drive (600Ω)
Of my three AKGs (K271S, K240M, K280), those are my favorite.
They are very comfortable and also smaller and lighter than the closed-back circumaural AKGs. Being 600Ω cans, they are somewhat dependent on the source: My old JVC CD player drives them well, so do the powerful headphone amps of older mixing boards. With less powerful outputs, e.g. digital pianos, they work ok, with portable equipment not so much.
Their sound meets my taste for general listening very well. Overall they are balanced and pleasant with reasonable attention to detail. Bass isn't as recessed as e.g. on the K271 - for listening that's a good idea IMO. Also, they lead to relatively little ear fatigue, although the don't match the K271 in that respect.
I was surprised to learn that although it is semi-open, the K240 Monitor is very dependent on the condition of its ear pads. Flat pads impair sound quality and comfort, as the baffles tend to touch the ear. Also, the new velours pads from AKG pretty much destroy their sound - so it's a good idea to stick to the leather pads, they're cheaper anyway.
As it seems to be a general consensus that those are great phones, it's hard to make a real bargain unless you're very patient on eBay. Also, it makes sense to replace elastics, ear pads and the foam disks on the baffle right away - it really makes a difference in comfort and sound.
Anyway, considering that I paid about €60 for my unit, it is a fair price for cans that serve as workhorses and for relaxed listening as well. As I said, of the three AKGs I have, those are my all-time favourite.