Well comparing it to the Mac Mini isn't terribly fair since it's arguable that the Pegasos is overpriced in comparison to the Mac Mini as well. Not as overpriced as the A1 of course, but neither are exactly great deals.
I don't understand why it's so important to you that the folks at Hyperion declare OS4 finished. What the people in charge say about it's level of completion has no effect on the user experience. I might question their business sense, but I suppose I might question the business sense of anyone trying to revive the Amiga platform in any way shape or form.
The cache coherency bug is unfortunate, but it's not a showstopper (at least not as far as OS4 is concerned, it's a problem if you want to run PPC Linux though). Obviously manually flushing cache lines before doing DMA is less than ideal, but it's not like there aren't other platforms with this limitation (the SH-4 for instance doesn't support hardware cache coherency at all and it did a reasonable job in the Dreamcast).
On a personal level, the showstopper is the price. A1 boards are just too expensive for my tastes (the lack of availability besides second hand sales is a bit of a damper as well). Hopefully Troika's board will be out soon, but they haven't set a release date.