As a Blackburn fan, I remember Man City fans 'invading' our stadium and town during the final day of the 1999/2000 (they surely had at least 15,000 fans in the stadium and several thousand more outside it), as they clinched promotion to the Premier League. That was one of the 2 most impressive away followings I've seen at stadium, alongside Celtic's in the UEFA Cup in 2002/2003.
During the Maine Road and 'typical City' era, they were considered to be one of the most passionately supported clubs in the country, including getting very good crowds when they were relegated to and then played in the third tier in the late 90s (and Stockport were in a higher division than them), dancing in conga lines singing 'bring on Macclesfield'. And to be fair they were getting crowds of 40,000 when they failed to score in 13 out of their 19 home league matches in 2006/2007.
Their relatively muted and dull celebrations after winning the each of their 3 trophies this season including the Champions League this season, seem like night and day compared to their euphoric celebrations after secured that promotion in 2000 at our ground (the fact that we lost that game 4-1 despite completely dominating it and creating so many clear cut chances also stood out)
This has probably already been quoted elsewhere, but this quote from GQ magazine summed things up for me:
'The Man Utd treble of 98/99 was something like a dozen miracles happening at once. Watching City do it this year has been like watching an office administrator fill out a slightly tricky form.'