Speaking as someone who spends upwards of an hour on your site daily (and, as an aside, thank you - PCPartPicker is an absolutely stellar utility, and I don't know what I'd do without it), these were my primary issues with the update:
Having the most commonly selected components along the top was pretty convenient. It's not a big issue, but the current top bar adds an extra click every time I want to add a CPU, motherboard, GPU, PSU, case, RAM, or storage. That adds up to a lot of extra clicks over time, if you design a lot of builds per day.
Having the filters along the right was more convenient, from my perspective. Since the most common buttons I'm going to be hitting are "view" and "add", having them directly adjacent to the filters bar made for a little less travel time. Again, very small thing, but I design a lot of builds, so seconds add up to minutes eventually for me.
This is the big one: breaking the list of parts up into pages. While the other two "issues" are really quite minor, this one actually does annoy me quite regularly. I know that most users probably aren't looking to load every case or CPU or PSU in existence when they click on that tab, but I find myself missing the infinite scrolling daily when I'm sorting through case options, PSUs, and other parts with a lot of, er, chaff in between the wheat.
I also didn't like the new aesthetic as much as the old one - particularly the fact that it appears that the text has grown slightly larger, which makes for a bit more scrolling when scanning a large list of parts - but that's as much curmudgeonly opposition to change as anything else.
Overall, I continue to find the site an absolutely invaluable utility, and I really appreciate you folks for making and maintaining it, but if there were, perhaps, a toggle on things like the side-bar position and the infinite scrolling, it would definitely improve my user experience significantly.