Every season has its good episodes as well as more than its fair share of crap. Also TOS and even TNG to a degree, are pretty dated and can get really ridiculous. It would be ashame for you to toss your hard earned cash away on something that you might not like. Having grown up with it I have a pretty good tolerance for the cheesiness that much of ST is, but I can imagine a new comer finding a lot of it painfully lame. Every episode is kind of hit or miss, same thing for the movies. The movies I like I really like, and the ones I hate I really hate. The Wrath of Kahn is one of my favorite Sci-fi movies of all time, while sitting through ST: The Motion Picture has been the way I always imagined hell.
Anyway, good luck on your quest to watch all ST. I will be looking forward to hearing your impressions of the franchise.