Control4 is a hardware based automation system. It is generally installed by integrators only (although I am sure you could DIY a system if you put enough effort into it). On a general level, it is similar to Crestron. Although I don't really know a lot of specifics about Control4, I believe that they offer a lot of hardware pieces, like an audio module, a lighting system, etc, etc, etc.
I use CQC which is a software based automation system. It doesn't have any hardware modules. This can be good or bad depending on your perspective. There are many audio device, lighting systems, etc that will work with CQC, but sometimes the wide range of systems that work with it can be overwhelming.
Control4 is an affordable hardware based family of automation products. They integrate with a (growing) number of other hardware vendors but also have their own bits of gear. Lighting, HVAC, media distribution, etc. They are the also the current leader in the Zigbee space. In the past, the gear was incredibly unreliable but, C4 has made great strides there and is now offering product that appears to work as advertised. They are also soon to release a Flash development module that will, in theory, allow people to finally be able to truly customize the system (C4 and HAI are on similar paths in this respect. Traditionally, they have both offered "locked" down interfaces). The past, I think, 6 months has seen the company doing a lot right and they are gobbling up market share like crazy.
It is possible, though rare, to DIY a C4 system. (see www.c4central.com) sic's right, it is a generally "pro" installed system. I put the quotes around the word pro there for a reason which leads me into the con's...
The primary issue that I take with them (and this reflects my opinion rather than that of the company I represent) is a perceived lack of integrity. The ownership comes from an HA background with a company called Phast that ran their business into the ground and took a lot of integrators with them. They literally bankrupted good, hardworking folks while cashing out and "running" with the money they could pull. There are a lot of dealers who still have a very sour taste left in their mouth from that debacle. In addition to this, the company's marketing arm will say anything to make a sale. By anything, I mean that they will publish complete and utter falsehoods. My favorite is that they claimed in 2008 to have been the home automation system chosen for the first LEED Platinum rated project in the US. I know that it is either a complete lie or miserably ignorant (neither is a good proposition) as I personally commissioned a full home automation system for the THIRD Platinum rated project back in 2006. I called them on it and they never rescinded. They allow misinformation to remain "out there" in order to bamboozle people. There are a number of other examples of delusions of grandeur but, they're easy enough to find if you dig. Lastly and, going back to my earlier "pro" comment, they are gathering a flock of dealers that is huge. In particular, they are gaining traction with those who are brand new to the home automation space and have dollar signs firmly affixed to their eyes. While anecdotal, I would suggest that a large portion of their dealer base has very, very little experience in home automation and yet will present themselves as experts (akin to getting electronics advice from the zit covered 16 year old kid at Radio Shack). Don't even get me started on their presence in big box stores and their "highly" (as in two week classroom course) trained install crews.
To sum up, Control4 is definitely a leader in home automation but, while they have gotten much better in the recent past, I would state that it is based on price point rather than reliability, ability, integrity or otherwise. I have recently begun to think of them as the Dustbuster of automation. Not very powerful but capable of "picking up" a large percent of simple jobs...