I think Lion is meh. But I upgraded since it's a lot easier in the long run to stay current than to make lurching changes later.
I think that there has been more criticism of Lion than other upgrades for a few reasons.
First, it's a change to have to go through the App Store. And selling machines without disks and forcing folks to use Recovery HD and/or internet recovery is a big change, and has a lot of detractors, justifiably so IMHO.
Second, it eliminates Rosetta and deprecates lots of dying applications. It was inevitable, and this just happens to be the upgrade where it occurred. People using orphaned software were gonna complain at some point in the future; it isn't Lion's fault per se.
Third, some hardware running on it does seem to have systemic problems. Wifi, battery use, etc. Apple is a big company that never admits mistakes, and while it demands a premium for quality doesn't always deliver it. People are gonna complain, justifiably, when they pay twice as much for a computer that has software/hardware faults.
Fourth, Lion was an "upgrade" but it didn't really upgrade much, just changed it. Spaces is a good example; only a few see that as a positive change. When a piece of software just gets changed for no apparent reason with no apparent demand for that change, then people are gonna wonder why they paid for it. The Mac OS hasn't had much of anything in the way of innovation in a long time and people aren't going to want to keep paying for just rearranging what's already there.
Rob