I can't vouch directly for or against Tokyo Marui, but my general understanding of their products is that buying from TM is like buying from Canadian Tire. It's cheap, it's strong, it's reliable, and it comes with lots of fun extras, but there's a chance, with every purchase, that it will just be crap and die before its time.
I know that TM weapons are easily upgraded with things like metal kits, internals and so forth. However, once you've shelled out the extra cash for the upgrades, the whole package costs as much as a higher-quality rifle would in the first place. So if you want a top-notch gun fast, you're better off trying something else, just to be sure.
But if you're looking for a long-term project, something to work on and play with, I don't think you can really go wrong with TM, especially as a starter AEG.