I have the AKG Y45, and when I first got it, I found them to be ok for out and about uses...not bad and nothing special. However after getting my Sony MDR-1ABT, I've put on the Y45 again to try, and just couldn't bare listening to it more than a 1 minute before putting it back down...the MDR-1ABT simply sound SO much better than I couldn't stand going back to the Y45.
This does highlight why cheap Bluetooth headphones are really not all that good. Money that is spent on the electronics, is money that is not spent on what is most important in a pair of headphones. Not so much of a problem when the Bluetooth headphones are expensive enough to take the cost of the electrical components, but have enough money left to make the headphones sound good too.
On the other hand though, it does depend on the individual. Two people who both own Y45; one is still happy with them despite owning better headphones, the other finds them too inferior to use after using better headphones. Although, there is a point when Bluetooth headphones become too cheap to sound any good to anyone whose hearing actually works. I think the Y45 are around £70. The Y40 is £40-45 though, and that is probably bordering on just a bit too low really. Anything lower will surely sound like toilet. Sure the Y40 gets good comments on the jungle site, but many people have no idea as long as the product actually does work.
At the budget end, wired headphones are by far the better choice from a sound quality point of view. If we consider the Creative Aurvana Live! for example, they will still sound good compared to headphones costing 3x as much. Maybe they won't sound as good, but at their low cost, they aren't impeded by being stuffed full of cheap electronics, just so they can be wireless.
If you would prefer wireless because wired is in some way or another a hassle, then get them from a place where you can return them if they aren't as good as you hoped for.
If wired is not a problem though, then I'd probably go for the Superlux HD668B. For sound quality, they will smeg over any similarly priced wireless headphones.
Edit: As above - if you can stump up the extra £10, the CAL! is well worth £50.