I've been using Carrara since 2. Moved to Cinema 4D because I wanted to move up to the next level, it's now my main 3D ap. Still, I plan to maintain upgrades for Carrara because it is an amazing package for the cost at around 1 fifth the cost of the full studio bundle for C4D and a comparable feature set for Maya the value is simply astounding.

Although the level of control, fine tuning, and high-end capabilities is greater with C4D and Maya (much more robust particles and dynamics, cloth generating, hair, expression for node based programming, more/better animation controls, etc.)

Cararra still actually does some things better (or at least a lot easier). Network rendering allows for multiple machines to render a single frame where C4D will only distribute complete frames (unless you jump through some pretty stupid hoops). Things like a terrain generator and a plant generator are only currently obtainable in C4D via yet again more costly plug-ins. I like the way Carrara creates single volumetric clouds it seems easier to me than C4D. Poser is not really supported (at least on the Mac) for C4D except though .obj with no direct transfer of animation from Poser to C4D. Carrara has great vector based flash options available that simply aren't there for C4D.

C4D and Maya are overall leaps and bounds ahead of Carrara if you need complex animation and overall more sophisticated and varied control. However, for the most part Carrara can do most of the same things at a level that is perfectly acceptable for small studios or individuals at a fraction of the cost. C4D does have an incredible community of people who write tons of free plug-ins as well as for fee.

All in all I highly recommend Carrara 5 pro. Compared to the more costly aps, Eovia is practically giving away the store for free. If after learning all of the things Carrara can do and you find you need more control, than look at C4D or Maya (IMHO, Maya requires a rocket scientist to learn). I'm guessing if you are wondering whether or not the upgrade to Carrara 5 is worth it, the jump up to one of "the big dog" aps will be ridiculous over kill.

Carrara 5 pro upgrade GET IT! You won't be sorry.