Sounds like a good deal. 15% is higher than unsual, and I have never heard of a guarantee to undercut the renewing premiums by 10%. Good advice would be to get it in writing and don't forget to ask about what will happen at the first renewal of the accounts rolled over.
I can't speak from the agency side, but its been done in soft markets at the companies I've been with for as long as I've been in the business, and its always good for a hoot at the next rate level review.
DAMMIT! You tricked me into giving a real answer! Now I have to shut up.