Basically, Karlos gets it right there.
AmigaOS 4.x is a brilliant OS, but on the BPPC or CSPPC it's little more than a toy. The other machines are the ones for which OS4 apps are written, usually, so it suffers on speed.
This is exacerbated by the amount of ports from the non-Amiga (i.e. faster) platforms there are for OS4, which makes the problem seem even worse.
That said, though, Amigas are for the most part toys. Sure, they can be used for serious things, but most of us use them because they're fun. So in that respect AmigaOS 4 on the BPPC/CSPPC fits the bill perfectly.
My A4000 on which I'm typing this is the only machine on which I can run OS4 and OS3 apps by dual-booting, and for that reason it classes as "cool" in my book. Sure it's not as practical as other solutions, so I would recommend it to complement a new AOS4/MOS machine, not to replace it.
Now if you'll excuse me, I think after having written this post in an OS4 browser, I fancy a game of slamtilt. On my Amiga, using an Amiga keyboard, and an Amiga AGA chipset. No PowerMac or AmigaOne can do that.