WarioWare, Inc.: Mega Microgame$! is very fun and surprisingly well-made. I found myself trying to beat high-scores and practicing individual mini-games to do so; i.e. grindy things that I wouldn't do in other games.
It might seem simple to throw together a variety of brief, interactive moments and call it a mode of play, but the design details really matter. If there were minimal thought put into the microgames, there wouldn't have been things like generous hitboxes and auto-correcting animations for some of the tight microgames. Most of the microgames have three meaningful variations of increasing difficulty. I have suspected that Mario Party and Warioware games are where junior developers' ideas are tested, where unfinished ideas end up, and where old work is recycled. If true, Warioware's microgames come together much better than Mario Party's minigames.
The whole idea of the game getting incrementally faster would seem to bring up technical problems with audio processing and frame pacing. I don't know enough about the Game Boy Advance or the Warioware itself to know if it's technical wizardry or just clever design.
If you've played any game in the series, you might expect this one to just be another entry, but I think Mega Microgame$! had two advantages. First, it lacks the control gimmicks of most of the later entries; e.g. while D.I.Y. on the DS has its own merits, it only uses the touchscreen. Second, the limits of Game Boy Advance suit the effort they were willing to put into the production of the game. Later entries are aesthetically similar, but their 2D (and later 3D) visuals come off as cheaply-produced.
I have two criticisms. Three or four of the microgames felt too hard to win reliably. Also, most of the "boss" microgames are too long; they break the pace of each run.
Beautiful, catchy, addicting. Play it!