I've never played Sonic '06 since I didn't own a PS3 around the time it came out, but the fact that game is supposed to be god awful did not help my view of the series. Sonic Unleashed was noticeable improvement but I got stuck with the Wii version at the time. I liked the game, but it could have been so much better if they scrapped the werehog and made some other changes. Fortunately, Sonic Colors corrected a fuckload of those issues and ended up being a really great game. I did try Sonic Rush on an emulator at some point in 2010 and thought the controls were so bad that it just ruined the whole game for me. Sonic 4: Episode 1 was fun but was just nowhere near as good as the Advance games or the Genesis games. Even if the physics were good for that game, I don't think it would have been that great. Certainly would've helped it a ton though.
One thing that should be kept in mind about the series is that it offers certain types of games I just can't find anywhere else. While Sonic was going down hill in the mid 2000's, the 6th generation of gaming was a pretty good time for fans of 3D platformers. I was absolutely obsessed with the Ratchet & Clank series, the Jak & Daxter series was strong, the Sly Cooper series had three very unique platformers, Super Mario Sunshine was awesome, I loved Rayman 3, and there were other enjoyable games from that era too. Once the seven generation of gaming hit, the selection of 3D platformers wasn't that great. Sure, the R&C series continued, but most of the games released from that series this generation have been nowhere near as good as the original trilogy. Super Mario Galaxy 1 and 2 kicked ass, but those were SD games and the lack of HD platformers taking advantage of impressive hardware was very disappointing. Game series like R&C, J&K, and Sly took great advantage of what could be done in a 6th generation console. Much of what was done in the Galaxy games seemed mostly possible on the GCN. Only 3D platformers this gaming generation that felt very 7th generation to me were the new R&C games and some of the Sonic games. Even then, I felt most of the level design from R&CF: ToD were very possible on a PS2.
Another thing worth bringing up is the Sonic fandom. While I didn't post on any Sonic sites in the early 2000's, I used to go to a ton of Sonic fan sites and lurked on quite a bit of Sonic forums. Unfortunately, the forums I used to lurk a lot disappeared overtime so by the time I was posting on forums in late 2004, those sites were no more. It took me forever to find a Sonic forum I liked but I eventually discovered this site's forum community and just had to join it. I was also part of Nushift's forum for quite a while had a lot of Sonic fans but the site owner deleted the forum last year. His original intentions for the forum were never successful and I think he wanted to completely separate himself from the Sonic fandom. I also used to read sprite comics a ton in 2002 and '03, and play Sonic fangames a ton in 2003 and '04. So anyways, this why I've stuck with the franchise. Certainly helps when my favorite game is part of it.