I watched season 1 of TNG on TV when I was about 13 (unfortunately, only season 1 was broadcast in my country), and I liked it, but it was responsible for me thinking for quite a while that Trek was kiddie stuff. Those early seasons of TNG really look naive and dated now - more dated that most of TOS, even. Later TNG is a different story, but still, DS9, for the most part, looks more mature and relevant. (Well, apart from most of the Ferengi episodes and a few other 'humorous' episodes.)
I've planned to watch B5 for a while. I've only seen "The Gathering" and the first episode of season 1 so far, but haven't had time to watch further- when I do, I'll watch the rest. So far, I like what I've seen. Well, "The Gathering" was so-so, just OK, but I really liked "Midnight on the Firing Line".
I agree with you about TNG. It took them a while to get their legs. At the time it stared airing, I was just so excited that they were renewing the Trek TV franchise that I was willing to wade through some not so stellar episodes to give them a chance to develop. I very rarely judge a tv series based on their first season, but if they don't start stepping up to the plate by mid-season 2, I usually quit watching. TNG did at least improve.
B5's weakest season is Season 1; however, once you've had a chance to watch the series, you're going to realize that there is way more
going on in that first season than it looks like the first go around. Unlike most of Trek, there are no real stand alone episodes of B5. It continually builds on the story, and it starts throwing in complex plot elements from the very beginning. In a way I envy you getting to go through it for that first time! If you love the Cardassians from Trek because of their complexity, then you are in for a real treat with the alien races of B5.