I had all my reels cleaned through APT in January, I asked about new bearings in all. Answer was with my Shimano Curados and Cores the bearings were already pretty good and wouldn't make a big difference, if any.
I "upgraded" the hearings in my Steez reels and didn't notice a difference at all. I upgraded and added three on my TWS Zillion and it was very significant. I added a level wind bearing and one to each handle. Made for a noticeablly smoother winding reel. The upgraded spool bearings added a little distance, and noise. But I went from stainless to ceramic.
In my Lews Team Pro, the bearings didn't last long. Upgrading them made an improvement, but I waited too long to do it, so of course it seemed like a huge difference. Probably is slightly better than new
My feelings? I like to tweak and obsess over my reels. For me, it was a way to have a little fun over the winter and keep myself in a fishing state of mind. Can't say the improvements were really worth it, but the fun I had working on them was.