Even though I love how it ends at ten years -- getting to see these characters' relationships evolve over a decade -- I think the show should have ended at season 8. Seasons 6 and 7 were lackluster compared to the first five, but season 8 was the last truly great season. Season 9 started off great then became hit-and-miss and 10 was just meh. I still love every season of the show for whatever reason, but the best time would have been during its Emmy-winning season, when it was the #1 sitcom at the time.

Season 8 would have been an ideal finish.
But, if they could maintain the quality of Season 9 and 10 (although not on par with the early seasons).... I could see the show going on for a few more years.

Actually, Season 6 had a lot of truly good episodes, including the two-parters from the second half. The story about Ross dating Elizabeth was a snoozer though, and I really like the clips eppie where Chandler doesn't write down the message about an audition for Joey.
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B.J.: "Here he is, our very own ray of sunstroke." Hawkeye: "Ever notice how his sneer lightens up the room?"

IMO, Season 6 started out great, but became hit-and-miss halfway through. I don't rate it as highly as I do the first five.

I agree. The first several episodes of season 6, with the Ross-Rachel stuff, was gold. After that, it lost steam. That is, until Bruce Willis came and performed "Love Machine" in front of a mirror.