I have used a few SIM tools and it really comes down to what vendors/devices the SIEM supports, whether or not you like the interface, what size your company is, and how much work you are willing to do to build rules.
For small to midsize companies take a hard look at TriGeo and Hightower (acquired by NetForensics). For large companies there are several good options, such as Loglogic, Novell Sentinel, RSA, etc.
I agree with the statements made so far on Cisco MARS also. I personally would not bother with MARS unless you are a Cisco only shop. There is some third party support, but it is not significant and is also not updated very often.