Great Dungeon Master, lousy writer, it seems.I loved Game Wizards, and similarly came away with a different opinion of Arneson (not to mention Gygax and Lorraine Williams).
Overrated is difficult to assess without context, but I do think Arneson generally gets more credit than he deserves. He definitely was the first Dungeon Master and came up with key ideas, like levelling. On the other hand, he did so as part of a gaming group that was sort of collectively building towards such concepts, and he was not the first in that group to act as a sort of DM-like referee for war gaming, if not specifically a fantasy dungeon crawl.
Nevertheless, it is indisputable that Arneson got the thing working that Gygax was able to turn into D&D, and so no Arneson, no D&D. However, after that his contributions diminish rapidly, and while he typically blamed others, especially Gygax, for this, in reading the actual records it is pretty clear that Arneson had trouble actually producing material. And not just for TSR - he had contract after contract in which he basically failed to deliver what he promised as far as actual work.
Ultimately, he comes off as a great ideas guy who wasn't very good when it became time to produce.