Found this interesting article in a new infosecurity magazine, on the demise of Cisco MARS, entitled "Where on Earth is MARS?"
The article references MARS past, and surmises on the demise of Cisco MARS, and continues to relay some of the negative sentiment from a handful of analysts in the past year.
I have to say that many people though appreciate and still utilize the many innovations and capabilities that MARS offers.
While a few SIEM vendors have incorporated some of MARS features, MARS is still quite a capable Cisco-centric monitoring solution.
That being said, I also do agree that if you have outgrown your MARS appliance, need to upgrade, require broader device support, and want newer features etc, then it makes sense to look beyond MARS and kick the tires of SIEM alternatives.