It starts to get really interesting on your 50th and 75th, as then you can start getting some serious OBC to work with!
Cheryl is a couple cruises ahead of me, and used her 50th on a chartered music sailing (VH-1 in 2011). I was surprised that they allowed it to apply to that cruise since the fares are substantially more than regular rates for those types of cruises. So that's something to consider since casino offers don't apply to them.
I forget the exact amount, but on the Celebration earlier this year we cashed in our on our 75th milestone credit. Between that and $2,000 on board credit from a casino offer, we ended up with several thousand in cash to use, which we used up easily (steakhouse for something like 13 people the first night as well as jewelry store and casino visits).
Tom