Since I've been able to upgrade to Mountain Lion,I was hoping for a seamless transition from Snow Leopard to Mountain Lion in terms of having apps that work 100%.Sadly this hasn't been the case for the official client.
Issues spotted that the dev team should fix soon:
1.Mountain Lion complains that the 'Bitcoin.app can't be opened because it is from an unidentified developer'.Remedy to devs-Get this app signed with a developer ID
User remedy until fix is applied-Go to system preferences>Security&Privacy> and select 'Anywhere' under 'Allow applications downloaded from:' category. Users who are stuck with defaults will assume that the client is a harmful app,even when it isn't because of Gatekeeper's warnings.If the app was launched before,the warning won't appear,thankfully but still, this could alarm some inexperienced users though.
2.The client stops responding when first launched everytime in OSX 10.8-I don't know why even though the whole OS is much more responsive than Snow leopard ever was with the client (0.63).Out of all apps on my newly upgraded (OS update only) MBP (MacBook Pro),the bitcoin client is the odd one out with the most crashes,non-responding and long delays before it finally loads.
3.Maybe optimise the client for OSX 10.8 while we're at it so that it's just as responsive as the rest of the system.
4.This problem seems to affect other clients as well so an advisory to the devs of those clients may be appropriate as well.
Thank you and I hope this helps you gauge the level of compatibility of the official client on OSX 10.8 (Mountain lion) so far