Parking at Assembly right now will be extremely annoying given how popular that area is but it's free and there's lots of it. We live in the south end, so we always try to take the T because I hate driving there with every fiber of my being.
But the theater is nice. About 8-10yrs old. There are much newer theaters in the city so we mostly stick to those but there's no reason to fear rats or anything like that. Assembly is your garden variety pre-fab outdoor mall filled with suburbanites.