I personally outgrew their products within weeks of using them. I find their base functionality good and solid but nothing really ever extends beyond that. Maybe my experience was that their product simply didn't fit my needs but I felt it was always missing something that they never included.
I always felt I was going to go back in a year and it would be there. It simply wasn't.
You are not the typical user; what makes 37Signals so good is that they clearly understand just how dumb typical users are and design apps normal users can actually use without having to get their tech buddy to explain it to them.
And honestly, it's not dumb enough. Email dominates the workflow of most businesses because that's about the complexity most users can handle, write mail, send button. I've seen people use Basecamp for weeks and still be confused by the fact that there's more than one tab.
Maybe they just didn't need to change anything?
"If it ain't broke, don't fix it."