I played Terranigma for the first time a couple months ago. It's pretty good. Gameplay-wise I think it's the best of the Blazer Trilogy. But, overall I liked Illusion of Gaia more, mainly because of it's cast of characters. Terranigma is pretty much about the hero with a couple other minor characters, nothing compared to the group in Gaia.
It seemed to be less connected to the other games also. It shares some themes and whatnot, but Gaia felt far more directly connected to Blazer than this does (especially if you get all the gems in Gaia so you can fight the hidden boss). But, Terranigma is tons of fun. It has a pretty robust set of armor and weapons that you switch between based on the situation and is a bit faster paced than the previous games. Also has a bit of an equipment system, but for the most part you find the equipment and equip it. So, if you need to get under water, you find the item that lets you go under water. Not like Zelda where now you have a boomerang or hook shot with multiple uses. It's more like, "I need to go under water. Now I can go under water."
Overall I'd personally rank it in the middle. Liked it more than Blazer, less than Gaia. They're all three pretty awesome.