+1 for ares. I'm a tavor owner and I love it. I've only had 3 issues with it.
-The mag release was too easy to hit... I kept bumping it with my gear so I cut about half of it off on the field. No more problems hitting it.
-The fire selector is annoying. I constantly bump the ambi-indicator when readying the weapon, which moves the fire selector part way between auto and semi... which in turns causes it to not fire, until I notice and bump it back into place. I'm sure there is a simple fix for this.
-Magazine compatibility. And I want to expand on this because I seem to see this come up a lot. This is NOT specific to Ares, despite what some of the more ignorant people on this forum might have you believe. This is the kind of problem you run into with all makes/models of airsoft guns. Tolerances in airsoft parts always leave much to be desired. With any airsoft gun this normally just turns into a process of elimination. I've only tried 2 makes so far. My greenbox pmag's have feeding issues. I got some King Arms mags which seem to be working better. I'd still like to try a few other magazines.
Unfortunately the King Arms mags have feeding problems with my KWA M4, (they also didn't work in a VFC M4 which is how I ended up with them). This whole thing about proprietary parts is rubbish. These guns are just as upgradable as others. Anybody saying otherwise is not worth your time.
Finally, Ares has great externals, AND great internals. The most important thing I think you can do is ensure that ANY Ares product you buy is the legally imported Canadian market version. The "Made For Canada" Ares products are built to run 400fps. They don't just toss a heavy spring in and then ship them out of China. They specifically engineer the gun to run at 400 fps, reliably. This is the same thing KWA does with their products. So when you replace the spring you're basically getting a fully upgraded gun. The only downside would be that because the gear ratios are designed for maximum torque you you'll have a slightly slower rate-of-fire than equivalent guns. I could say more but this is already far too long.