Impossible to give you a decent answer as you do not say what your current position and prospects are. I will therefore give you a highly biased and indecent one!
Cobol has been around for many many years, in many many guises and flavours, and will continue to do so for many more decades. Cobol seems to me to be a constant that the IT industry relies upon. If an IT innovation comes along (Relational database, client server, Web technology etc.) it's not long before work has begun on getting Cobol on board, thus ensuring that Cobol lives on. The many different platforms, operating systems, and software, that Cobol is available/interfaced on, and being developed with is awesome.
Because of it's longevity, there are quite literally millions (billions?) of lines of Cobol being run throughout the world every day/month/year, and this is a testament to the strength of the language.
That said, because of the volume of Cobol software, and it's relative ease of use, there are an awful lot of Cobol developers out there, and because of the diversity of software/platforms etc. I feel that the Cobol market is not as boyuant as it was 10 years ago, but then again, is any?
My advice: If you are working for a company that is offering the opportunity of a Cobol starter role, and this is a sideways, or not too much of a drop down, move, then I would be inclined to go for it. I would also look out for other courses once you have got Cobol under your belt, as it is a good weapon to have in your technical armoury, but should not be the only weapon (if of course you wish your career to go in that direction).
Hope this diatribe is of some use, and I look forward to the views of other members of this forum.
Marc