I'm working my way through season 1 now. I'm not encouraged by what I've heard about the later seasons.
I've seen the show on DVD myself, so here's what I've gleamed about it, quality-wise:
: Great all-around, through it can be a little corny in spots.
: Good, but some episodes will test your reality quotient a lot.
: Disaster all around-the show was on opposite the shitty Batman
show, so it's plots were neck and neck with the crappy plots of the Batman
show (the episode where Solo & Kuryakin have to prevent a stink bomb from hitting Las Vegas is one of the series's lowest points ever.) Only good episode is the one about the man running for POTUS (guest starring Richard Anderson [the future Oscar Goldman] as said candidate, whose wife is kidnapped and replaced by a clone.) Show was headed towards cancellation as a result of this dick move.
: Back to what Season I was, but way too late to save it due to the foolish excesses of Season III; show was canceled.
'The battle (between good and evil) ends once and for all in favor of good.'