Yes it can be useful.
Career in SAP is good both in terms of the salary structures and career growth. However, it depends on your interest and career objective. If you are interested to make your career as functional consultant of SAP then SAP FICO would definitely help you.  
SAP FICO refers to a specialising in two of SAP's functional modules FI Financial Accounting and CO Controlling (cost management). You would need to have 3-5 years of "domain experience" i.e. to have worked in Finance or Banking to find opportunities as a functional consultant. There are many opportunities for SAP Certified professionals with leading IT organizations which are into SAP consulting solutions worldwide.   About salary expectation, it is extremely difficult to give you exact figure of salary offered in any organization as salary depends on the different parameters like place of work, size of organization, roles and responsibilities etc. 
All the best!