My testing was limited to LFR, but Darkmoon Card: Blue Dragon seems to have up to 50% uptime potential. Here is the pudding:
Tortos, 49% uptime: http://worldoflogs.com/reports/rt-ua...?s=2235&e=2701
Megaera, 44% uptime: http://worldoflogs.com/reports/rt-ua...?s=3608&e=4225
In my crap gear (holy is my second spec) The difference between normal and in-combat regen is 6k mp5. So this translates to between 2,6k and 3k mp5 gain. Your mileage may vary.
Comparing this to, say, the valor trinket, which has 828 mp5. We gain between 1,7k and 2,2k regen and lose 1467 int.
Admittedly this is where it gets murky, because I cannot dig up the values for how this regen translates into spirit, so we could compare int to spirit which would be more intuitive. However I have a hunch Blue Dragon could be quite powerful.
Edit: If a random wowhead comment is to be trusted, 967 spirit equals to 545,8 mp5. Which translates to 1 mp5 is worth 1,77 spirit. If this is true, Blue Dragon (in those two cases) is a gain of 3009 and 3894 spirit compared to the valor one.
Do note this may vary with your class, healing style, raid size and especially your regen-combat regen delta.