ARES/Star's SA80 range is perfect externally. You'd be hard pressed to find a difference between one and the real deal.
Internally?
Not impressed. Credit where it's due, they made the first successful SA80 AEGs, much like the TM FAMAS (before that, you had the Academy L85 plastic contraption...). Taking them apart is a nightmare, something I'm glad I've only watched and not had to do (and was the deciding reason for me taking ICS instead).
For example - on ICS/G&G, removing the reciever take-down pins and splitting the chassis allows you to get to work on the barrel/hop/gearbox. This isn't the same for S/A - to start working on the gun you need to undo a fair few screws and bolts.
The LSW suffers from a bendy/flexing barrel, as it is very loosely fitted to the chassis.
When they're running properly, they are a joy to use (aside from above LSW issue). But if you get a lemon (which is possible, this is S/A afterall) - boy do you have a lemon.
Armalites? I would say TM or G&P.
Wide choice for parts, fairly reliable and good to use (I really enjoyed using the G&P M16A3).