I am currently working as a MySql Database Designer (2+ years of experience, earlier in MS SQL) in Some Company. I am getting an oppoortunity to work with Informix Database in a big company. Will it be a good decision to shift to a new field? How's the career in it? Can anybody show me the light?
Sure, getting into another platform gives you some breadth of knowledge. Getting to know the comparative differences between Informix, MySQL, and MS SQL will only make you a better developer. If you're asking if it's a good idea to change careers long-term and become a dedicated Informix developer, I'd probably say no, though, unless you could get some very, very deep and specialized skills in it. Then you could eventually charge a bunch of money as a consultant to the poor saps that are using Informix these days instead of MSSQL, Oracle, or DB2...
See the questions Informix future and (less clearly) Upgrading Informix or Switch to Oracle or Sybase for my answers to previous related (but not identical) questions.
Having knowledge of more DBMS will only enhance your abilities. Understanding what's common to them all, and understanding some of the differences will make you more versatile, and hence valuable. Few companies run a single DBMS everywhere - even when they claim that's the case.