Well I'm glad to hear that some folks who hadn't considered SMAC before are willing to give it a try. I agree with some of the earlier comments that point out that SMAC has a bit a of a learning curve.
The basic concepts of game play are pretty much the same, but what makes the game more difficult to learn is that the technologies, wonders, base facilities, etc, all have strange and unfamiliar (to Civ veterans) names. Once you get used to the different naming conventions though most of your Civ skills are directly applicable. (command center == barracks, network node == library, and so on.)
SMAC's combat system is the best of all the Civ series that I've played. (I didn't play any of the CTP games.) The diplomacy is about equivalent to Civ3 but I think I almost prefer Civ3's diplo to SMAC's, but only just barely. Wonders (called secret projects) are useful and distributed evenly throughout the game, and as I mentioned earlier it's actually possible to win in a variety of ways on most any map. Overall the game has the "just one more" turn factor that I never got from Civ3 and the replayability is very high. One other thing that enjoy about SMAC and I think is overlooked in Civ3 is that SMAC provided a wealth of useful information about your faction (and others if your spying was successful) in a pretty usable and informative format. I think Civ3 wasted a lot of screen real-estate trying to look nice that would have been better put to use displaying critical information. The integrated story line is well thought out and immersive.
On the down side, while the AI in SMAC is better than anything that predates it, I think Civ3's AI is a lot better. A lot of the keys that we all grew accustomed to using in earlier Civ's were remapped for reasons that escape me. (sentry = l?) Many people complained that the tile-set used for the maps was ugly. (I agree with that sentiment but it is ugly for a reason: it's a hostile barren ugly planet that you can do something about if you don't like it.) Some units, especially near the end of the game, are very over-powered. (The 'copter units which have as many attacks as they have movement and the fly-anywhere-capture-anything locusts of chiron come to mind.)
Civ3 gets dinged for not having MP, but after seeing how difficult it was to get even decent MP functionality into SMAC I'm glad that Firaxis are taking the time to (hopefully) do it right on Civ3. (Anybody remember the disaster that was Civ.NET?) MP in SMAC works but be prepared for a very very very very very very LONG game.
On the plus side for Civ3 I like the resources idea; I think it's a great innovation. I'm disappointed though that as a tradeoff loosing cities in Civ3 has been made much less painful. In SMAC you work VERY hard not to loose cities because it makes such a big difference. In Civ3 protecting your resources is more important than your population centers most of the time.
Anyway, I hope that a lot more folks give SMAC a chance and find it to their liking. As someone earlier posted I'll be happy to share what I know about SMAC with any newbies.
Jared