I have started with Tamiya kits personally and have really enjoyed working with them so far. I have been working in the 1/48 scale though. I seemed to frequently read about the overall quality of the kits being very good so figured I would run into minimal problems with fitting, etc. This has proved to be true so far.
As far as ease of assembly and such, I find that there is just enough going on that I feel challenged and motivated to research techniques that will assist me often. This has not yet been overwhelming at all, though. It's been just challenging enough to make it very enjoyable for me since I love reading and learning about technique and application just as much (maybe more, lol) as building itself.
It seems to me, and this is probably a pretty subjective thing, that the primary difficulty thus far is in the finish. I can get a model together well enough and looking good (quite possibly due to the quality of the Tamiya kits I bought) but the hardest and most fun thing I've been doing and trying to do, is get the finish looking really good. I think this is probably true in any scale to a degree but stands to reason that the difficulty increases exponentially with scale since, obviously, more detail is shown.
I have a couple Tamiya A6M2 kits, one is almost finished, the other unopened. I grabbed a Bf109 E4/7 Trop that I haven't started yet. Finally, I have an A6M3 that I'm right in the middle of.
The three Zeroes I believe are older versions/OOP kits that my local store had left on the shelves. The quality of these are good. The Bf109 is a current production kit I ordered from Tamiya and I can say that the quality of that kit is visibly much better than that of the older ones.