Coming out of a long period of lurking to comment on my favourite band...
I do think their consistency is remarkable, but so is their longevity given the constant turnover of musicians. I suppose it helps that most of the departures seem to have been amicable, with people leaving to focus on other things, so it doesn't appear to be a case of a tyrannical band leader who's difficult to get along with.
What's probably also helped is that even though their debut was released in the early 90s, unlike most of the other prog metal bands who emerged at that time, they weren't trying to sound like Dream Theater. While they've got the chops, it does seem like there's always been more of a focus on crafting memorable songs than showing off musically, and that's given them their own identity rather than being just another clone, and may be what's kept people interested in them.
As for favourite albums, I would actually go with Legends of the Shires first, followed by Subsurface and then March of Progress.