Fair warning: 7zip support in 0.147 is trashing memory

: preg_replace(): The /e modifier is deprecated, use preg_replace_callback instead in /home/benbruscella/www.messdev.org/includes/unicode.inc on line 311.

On Mac OS X it’s nice enough to crash right away; on Windows/Linux/BSD it’s affecting god knows what inside of MAME/MESS/UME. You should stick with plain .zip if possible until this has been resolved.
(Also to be clear: I wrote “0.147″ above, but this includes all versions since .7z was added).
UPDATE: This problem has been resolved in current SVN, so if you build from there it should be safe to use .7z ROMs (although obviously this points out that they haven’t gotten much testing at all, so you may want to keep your .zip collection as a backup). If you aren’t building from SVN, the fix will obviously be in u1. Note that 32-bit builds are probably safe, but 64-bit builds on all OSes *did* malfunction. If they did so invisibly (it seemed to work for you) then double-check your .cfg and .ini files to make sure they weren’t trashed and consider yourself lucky.
Note that even with this fix, .7z support is still considered experimental; it is irresponsible for ROM sites to offer .7z sets/torrents without also informing you that by using them you are participating in what is effectively early alpha testing of a new MAME feature (none of the devs run this format regularly, for instance). As we’ve found with CHDMAN, there is often significant value to letting other people find out what’s broken.