I guess it's a matter of taste, but I thought the clear and practical iView interface was made less useful by Microsoft's - to my eye - crude reworking of it.
You are right that Phase One's interface is also very unfriendly - it LOOKS good, but the spindly gray font reversed on black is almost illegible.
The rendering time for images is sluggish. Large files do indeed appear sharp, but for some reason jpegs are rendered soft, and exported email size images are likewise. However, after reading an earlier post, I just reset the preferences to Apple rendering engine which seems to improve the performance and yield a sharp image on screen.
Yes, iView 3 was the best-looking incarnation. They really created a "best-in-class" application. Great attention to detail, incredibly usable. I upgraded to Expression Media for the larger lightbox (6 images) and, believe it or not, greater stability (for me at least). I don't mind the minor interface changes. To me the real "crime" is Phase One's apparent lack of interest in Media Pro (in addition to their horrid interface changes); the last two minor updates were a year apart. They say they haven't abandoned it, but they acknowledged to me over the phone that their developers are focused on Capture One, not Media Pro.
For what it's worth, I plan to use Expression Media until I'm forced to find another program. It appears to work on Mavericks with only a couple glitches. I would suggest you re-consider Expression Media if iView 3 won't run on your Mac. I think Microsoft's interface changes are too trivial to cause a person to refuse to use the software. iView's excellent functionality and usability are still intact.
I haven't heard whether EM works on Yosemite. That could spell my downfall.