I just ran Mail.app (5.2) for the first time on Lion (10.7.4).
All went well, except "Identifying message conversations" took 7.5 hours (5.5 hours CPU) instead of "a few minutes".
The progress bar looked stalled, but moved about 1 letter every half hour.
For example, from under the letter "t" in "take" to under the letter "a".
Envelope Index grew to around 40MB. I think my mailbox was around 200MB