Peter Boughton has been spending some time creating a comparison chart(wiki) for the ColdFusion engines out there.The new site is at http://www.cfml-engines.info and of course as a wiki you are welcomed to update where applicable. So far the table compares 5 engines which are:
ColdFusion 8
Railo 2
OpenBD
BD J2EE 7
BD.NET 7
You get a good overview of what you can and can't do but from what I can see this is limited to the functions only at the time (which Railo 2 seems to be ticking the most boxes). So have a look, update the pages and this will be another good resource for CF developers.And maybe even the CF engine developers could use this as a wishlist for future requirements :)