I own a TAR-21 by Ares. Here's my 2 cents:
- Poor air seal out of the box, can be somewhat remedied by changing the nozzle (M4 type)
- Proprietary gearbox and hop up
- Some issues with fire selector modes (sticks like a lot of other bull pups would) there's a fix that involves sanding a part inside the mechbox
- Poorly placed mag release (putting a stronger spring may help with this issue)
- Ergonomically, it feels good to me
- Crappy iron sights, non-removable as well
- Quick change spring is nice
- Working on the gearbox is easy (a small pain to remove it from the body, but once you get used to, its better than working on V2s)
- Issues with piston compatibility (due to the design of the mechbox)
- I like the finish, it feels durable, and I've put mine through crap
- Rubber on the butt stock wears a bit quick
Overall I had to put some more money into my TAR to get it up to par. Now it shoots very smooth.
Best bet for choosing a new gun is to go and handle them yourself (preferably stock so you know what you're getting).