I would like some advice concerning how well (or not) the new Roland jv1010 module might complement my existing Korg X5D.At $400 it seems quite an appealing opportunity, but then again I do want to avoid a GAS attack.

I have some interest in the JV1010 myself, so I checked it out. Its loaded with great sounds and would really expand your sound palette.
When I sold gear, I always advised people to consider buying units from different companies so as to give themselves more options. This is a prime example and good value-for-price.
Be advised that it has only a 2-digit LED display, allows for the addition of one sound card from the Roland library and requires a computer to fully program. However, the software for doing so comes with the box; factor that into your decision.
I'd say that outside of buying something fancier, the 1010 would compliment the X5D nicely. Both have onboard FX and its a great intro to the Roland world. Besides, you can always use the extra voices, sooner or later!
As both are 64-voice machines that grace you with quick response times, adding a sequencer would make for a really monstrous and portable setup.
Hope that helps some.