The last couple of episodes have been pretty good although they don't make up for what, IMO, was the weakest season of the show. It could, however, lead in to what may very well be the best season as it seems we'll be seeing the lead-up to the Observer future.
I've been watching the first two seasons on bluray recently.
(I haven't really watched this show previously, other than coming across random episodes via channel surfing while bored).
The first two seasons seems like an updated version of X-Files (ie. homeland security, etc ...), with some elements of the movie Altered States (ie. floatation tank) and some cyberpunk-ish elements (ie. giant powerful megacorporations running the world, etc ...).
The end of the first season and season 2 seems like X-Files meets Sliders.
I haven't seen season 3 and 4 yet, other than several random episodes.