I think that Civ IV definitely has more depth than V. I think that with V, they made it a little bit superficial to attract a larger audience. Basically, trying to attract new people who have never played Civ before. But the superficial design may be a turn off to die hard Civ fans. (But after the Gods and Kins expansion, it added a lot more depth and improved the game tremendously compared to what vanilla was.)
There are aspects of Civ V that I like better than IV. For example, I like the fact that there are hexes instead of tiles. It feels more natural than perfect square tiles. And I think borders expand in a more natural way. I also like the combat system of V better. I know many people don't like the 1 unit per hex, but I personally like it better. Once it a while it gets annoying, but overall I think it makes combat more interesting and strategic. The graphics are also better, simply becasue it's a newer game and designed for more powerful computers.
Overall, I'm happy with Civ V. Civ IV is also a great game, and III is a great game too (I can't speak for Civ I or Civ II, I never played them). I think that every series of civ is good in it's own way. Civ V and Civ IV are different games, that's for sure, but they are both Civ, and I think they are both good.
The one series of Civ that I did not like at all was Civ Revolution (the video game version). Civ Revolution was so superficial/simple/watered down, it barely scratched the surface of what a Civ game should be.
But anyway, I think that people should play whatever they enjoy the most. That's really the most important thing. But if you tried V when it first came out and haven't played since, I think it's worth giving another try, the game has improved a lot from what it was originally. Of course, it will always have that Civ V feel no matter what.