In my opinion seasons 1-5 are the best, but there are quite a few things to like in seasons 6-10, and are well worth watching. Things change; some actors come and go (or more accurately go and come back), and one slowly disappears over time. Seasons 9 and 10 are really a different show altogether, but still not bad. Stargate Atlantis is kind of set up by the events at the end of Season 7, so if you're interested in Atlantis at all, you'd benefit from watching it.
Nods Head vigorously. Although, a bit different and not quite as good, I thoroughly enjoyed S7 - S8, and thought it was amongst the best in "wrapped up SciFi Arcs" (For Me. Farscape was on the bottom of that list until Peacekeeper Wars
got greenlighted, and B5 was on top [Farscape, of course got elevated quite high after Peacekeeper Wars
S9 - S10, totally different show, felt like a spin off, but, I don't think it lowered the "Cred" of the Franchise, and I am glad to have experienced it, and I own it along with the rest of the Franchise, and when I watch The Franchise, I watch straight through, season by season, one show at a time (Though, SGU
S1 takes a certain mood to want to endure, so may be skipped from time to time on marathons [There are a few good episodes but, most of S1 SGU
wasn't enjoyable for me, it was a chore, waiting for something to endear me to the show, like more than 1 or 2 characters I didn't want to space, story that made me wonder and care what next...])