Well I can't really see what the "plenum" on the top of the unit is attached to. It looks like they just stuck the AC coil up against whatever was there instaed of putting on a new plenum and start collar. There is a PVC drain that is definitely pitched the wrong way. The return looks to be okay sheet metal wise. I agree with the purple PVC cement. That's why I always use the clear stuff but you're quite right, it's asthetic only, as long as it doesn't leak.
It's certainly not the prettiest job but then again, I have no idea whether you paid top dollar or got a bargain on the purchase price and since pricing isn't allowed on this site, you'll have to be the judge of that. To be honest, I wouldn't be very happy if my installers did a job with a result like that but we do charge top dollar so we can afford to put in the 'extras'.