I've been using my iMac 2012 21.5" with fusion drive for just over a month now.
I lived with the speck of dust behind the screen and out of the way, and I'm mostly fine with it.
Anyway, so far the fusion drive just screams. Boot up is snappy, and apps open with just one bounce.
The magic trackpad is awesome, and I found myself starting to do stuff I didn't think I would, for example, clicking with my dragging finger. Battery for the trackpad is good. I don't turn it off ever, so even when my iMac is shut down, it's still 'on' (probably a low power mode). So far it has only just started the low battery warning (10% power left), and it has been a month.
Keyboard is at 45%. Pretty happy with that.
I also shut my mac down at night now. Something I didn't anticipate moving from a laptop is that I don't have persistent power, and since it's so snappy on bootup, it feels fine to just shut down at night.
I have encountered a slightly weird bug with an external seagate drive that I'm using for time machine. Occasionally, my mac will sleep, and when woken, it doesn't see the drive, which has some how been dismounted, so I get the 'incorrectly ejected' warning. I don't think it's the mac, but maybe more the drive, as I had it happen once on my old laptop as well, where I never had issues before. I've heard that some seagate drives come with firmware that has power down modes, so maybe it is related to that.
I'm trying it in a hub (my cinema display) and it doesn't seem to happen there so fa, but the occurrence is pretty intermittent.
Why does my iMAc make a sleepimage? It is on hibernate mode 0, but if I delete it, it recreates it on boot. Further, I notice it doesn't really use it, since my last time it was modified was at a date and time exactly occurring with my last weird external drive dismount. Maybe it's related for the sleep, like the drive interferes with the sleep state or something.
I dunno. It's all anecdotal and minor.
But so far, really enjoying the iMac.