To be fair, it took a little while - I was a little wary at first, but it quickly grew on me. Once I got a feel for what was going on, and how the characters worked, yes. It grabbed me. I especially liked Jeffrey Sinclair as a leader-type - someone more thoughtful than your bog-standard action-hero.
Then, after a long wait, my friend lent me seasons 2-4. Again, I was a little wary at first; it had been a while since I'd seen season 1, and I knew that Sinclair had been replaced by John Sheridan. Out of the two leaders, I prefer Sinclair, and for the first few episodes of season 2, I was unconvinced. Then, about a third of the way in, the pace quickened up - considerably. And while I still prefer Sinclair as a leader, by about episode 8 of season 2, I found that I didn't have the time to miss him, things were moving that quickly, there were so many plots unravelling . . .
And therein lies the hook. The amount of detail is extraordinary, the way all the characters and plots became intertwined is amazing. The series is so intense - I was watching up to eight or nine episodes a day, for a week solid! And might I say again, I was a Star Trek fan who had never seen B5 before in my life! As many people before me have said, this show isn't a nice neat everything-gets-resolved-in-the-space-of-one-episode type. Details get carried over. There are long story-arcs. There are things happening all the time. Everything has a reason. You see an insignificant detail in season 1, suddenly it is explained in season 3 as being very relevant for reasons you didn't even know about back in the first season.
If there is one stumbling block for B5, I would have to say that some of the computer-generated images (CGI) are not always up to standard. This is especially true of planet surfaces, in particular Mars. But that's quibbling. The key to good science-fiction is believability. The characters have to act as if the basic premise of the series is perfectly normal to them. That's what makes it work. That's why in the original series of Star Trek, Gene Roddenberry made a point of not explaining the workings of anything. For example, Captain Kirk picks up a phaser and fires it without stopping to explain what it was or what it did - after all, in a contemporary series, nobody goes out of their way to say how a gun. We know how it works - so by watching Kirk fire his phaser, we understand implicitly that it is a weapon. The same principle applies in B5 - although to a lesser degree.
Moving on to the acting - this show was blessed with some wonderful people. In particular, Mira Furlan as Delenn is worth watching; she is possessed of a presence and an aura most actors can only dream of. She is one of those people that when she is speaking, becomes the absolute centre of attention without really trying. Also, as stated above, I am a fan of Cmdr Sinclair - and I think Michael O'Hare's portrayal of him is wonderfully underplayed. It would have been so easy just to play him as an action-hero, but no. I really do miss that character. Another favourite would have to be Stephen Furst as Vir - DS9 fans just think Rom, and you're halfway there. Not to mention the "odd couple" of Peter Jurasik (Londo) and Andreas Katsulas (G'Kar) - wonderful.
Actually, that's another point - the aliens. The aliens in B5 are more real than the aliens in Star Trek. Their agendas are much better fleshed out. We understand the whys and wherefores much more than the stereotypes of so many Star Trek races (and remember I'm a longterm Star Trek fan). To use Trek parallels, the Minbari are like the Vulcans and Bajorans combined in temperament, the Centauri are probably most comparable to the Cardassians, and the Narns, well, I'd say Klingons, but that's selling them short. Klingon-Bajoran, perhaps. Any hardcore B5 fans offended by those descriptions - sorry, I'm just using them as guidelines to the uninitiated Trek fans. These B5 races are much more real, less stereotyped. (Although the less said about the Drazi, the better!) To summarise, then - I think this is the best sci-fi since Original Star Trek. Watch this show! Star Trek fans will not be disappointed!