for quite a while now, 7-zip has been my major archiver program. it works quite well, isn't large and clunky, and even though its icons suck, is pretty good. i usually use it together with another archiving program - either ZipGenius or IZArc, though ZipGenius has nicer icons. that's because of the ACE archive format - for some reason, there's no support for this in 7-zip. IZArc seems to handle the largest number of formats of the three, though since i don't have it installed, i can't make any definite statement on that. in case you're wondering why i don't just use ZipGenius and ignore 7-zip, it's because 7-zip has the handy feature of extracting all useful information in an archive. that means if an archive is corrupt - say about 4.5kb from the end, when you extract from that archive using 7-zip, all the useful information from that archive up to the corrupt point is kept after it hits the error. other archiving programs delete the file after encountering an error, which may be shooting you in the foot, as even the partial extract can be useful.
anyway, i had some problems with all three. when you tried to extract a series of archives using their shell extensions, all three launched different processes for each archive handled, meaning that if you were extracting two archives using the ZipGenius shell extension, ZipGenius would run twice; for five archives, it would run five times, and so on. the versions of ZipGenius and 7-zip affected were 6.02 and 4.42 respectively (as far as i can tell, the latest version of IZArc still does this. WinRAR has had it right for about as long as i can remember). this may not have been a problem, but let's say all the archives had a file with the same name in them, and you were extracting them into the same location. you'd have to answer the overwrite dialog for each additional archive - which, in case you didn't already figure it out - is annoying (this is more common than you may think at first blush - lots of people put a file named 'readme.txt' into archives).
in addition to that, ZipGenius didn't work well on Vista - which i found out to my chagrin after installing it for a friend. the shell extension kept crashing Windows Explorer - so i uninstalled it and installed IZArc for him. after getting home, i went over to both the 7-zip and ZipGenius sites and got both the latest beta (4.55) and the latest update (6.03) for each of them, and installed it. now, 7-zip uses just one process, no matter how many archives you're extracting, and the shell extension is disabled on Vista for ZipGenius (haven't tried doing the multiple-archive extract with this update though). apart from that, 7-zip quits with an error message if it can't open a file as an archive instead of just opening the folder containing the offending file. also, the ZipGenius update has its shell extension on steroids - not only .zip archives, but .rar and even .tar.gz archives report how many files are contained - but this number can be misleading, especially if the archive is corrupt (like when i was downloading an archive and the download broke...)