But solid software, really? I give them one thing: it works. Can't recall encountering any real big bad bugs either, so yeah in that sense, quite solid.
But given that you contributed to it I assume that you've seen the code? I've had to make some modifications/hackery here and there myself and the amounts of time I've cursed at it for being a tangled and obscure mess... combine that with the childish language in the comments, urg.
I guess that's one important thing it taught me: never be flippant or rude in your code comments because it'll invariably end up making you look like a fool. Kind of like that "Muphry's Law[sic]" someone quoted in another thread. In "production code", but I found it easier to change my habit to "in general" because you never know who's going to see your code, and even your 3-months-future-self might as well be a different person when reading code is considered (except future-me does tend to share my sense of humour).
Because of that, all my own modifications ended up looking like rather ugly hacks as well because there was simply no "coding style" to join in step with.
(and the templating system (that is neither). and the amount of semi-duplicate subroutines and their names!!. "post", "post2", "message". And trying to guess in which of the five "/∗ probably the most important module of SMF ∗/" giant includes you're going to find a certain routine is about as predictable as trying to guess the name of a PHP library array function without reading the docs ... I really should stop, sorry :) )
It does work very well, I must repeat that. But only as long as the original magicians are still there taking care of it I'm afraid.