Parnering on a complementary product can almost always create a win-win situation. The only thing I would watch out for is the partner eating too much into your line of business or worst, being your competitor in the long run because he/she has realized how successful your venture is.

There is not much control on the latter, and like the saying goes, "whatever will go wrong, it will go wrong"

If you already have a very strong position in your own market, it helps to minimize some the risks hightlighted.

Best of luck.