I used Objective Zip and it worked quite well on iOS4. Work with this library is easy as this:
ZipFile *zipFile= [[ZipFile alloc] initWithFileName:@"test.zip" mode:ZipFileModeCreate];
ZipWriteStream *stream= [zipFile writeFileInZipWithName:@"abc.txt" compressionLevel:ZipCompressionLevelBest];
[stream writeData:abcData];
[stream finishedWriting];
Unzipping is done in similar fashion.
I can recommend it, but I can not provide any comparison to others. You can see on the wiki also some other libraries, which are based on this one. I hope this will help you a little.
