That's a nice deal! My calculations show that $15k would grow to an amount to give you a $750 per month withdraw rate under the following circumstances if you did it on your own:
1) With a 5% withdrawal rate, annual investment gains of about 9.3%
2) With a 4% withdrawal rate, annual investment gains of about 10.2%
If someone gave me the opportunity to invest $15k and get a guaranteed 9+% on it for 28 years, I'd jump at the opportunity.
That said, there may be other potential factors. For example, is you state's pension system in a good position right now? And is there any sort of inflation adjustment once your husband starts receiving his pension?