I've been running my C-250 for about 10 months. I'm actually very happy with it in almost every respect. Most of my reasons for considering another tank amount to "boys with toys". There is absolutely nothing "wrong" with it, but there are features I would like...in part just because I think they are cool, not because I need them.
I think the entire system is well put together. Lights, pumps, skimmer, sump, media rack, all work great and since it is all in one they fit well together and take up less space and look sleek. Even the "single plug" is quite nice. It's also easy to make the few upgrades I've made (ATO). As I've mentioned, I also love the closed top as opposed to an open top.
So, really I'm I'm thrilled with the C-250 as is. I'd highly recommend it to anyone.
The things I'd like that it doesn't have are:
(1) LEDS: I don't need these. The T5s work great. But, I want them. The Steve's LEDs work well if you are a confident DIY guy. I am not and the horror stories of some of the installs has scared me off. The instructions I find downright intimidating. Many others tell me it's easy after you are done, just daunting before you start...if you know what I mean.
(2) Size. 250 is a GREAT size. Not too big and not too small, but I wouldn't mind a tad bigger, but not a lot bigger. No more than 90 gallon. Too big and it just doesn't fit in my room well. I'm happy with the size, though. I'm not complaining. 90 gallon may even be too big. My wife would kill me if the tank took over the room. I'd look at it more and measure it out before I actually upgraded size. Upgrading from a 29gallon, I'd think that the 250 would seem like an awesome size. Of course, if I could really do it, I'd like to get one about the size of the living seas aquarium in Disney World, but that's a bit beyond my budget and size limitations.
(3) I like the idea of being able to add a more sophisticated sump in the base. Not that there is anything wrong with the rear sump. It can just give a bit more room and make it easier to add things like a phosban reactor or other things if I want to. Once again, is that a huge deal? Not really. Boys with toys.