We used to have problems with McAfee, but the program has been pretty reliable the past year or so. In a corporate environment (or anywhere else for that matter), I wouldn't run the "home" version 5.x; I'd run VirusScan v4.5 + SP1 + newest SDAT. Also, I'd definitely upgrade v4.0x or older if you're still running them. I just read on McAfee's site that they rolled their DATs back to 4109 because of some issues with v4.02 with NTFS. 4.02 is ancient. People still using that version (and probably don't update their DAT files weekly, either) deserve to have problems IMHO.
BTW, VirusScan scans for ~60,000 virii, worms, trojans currently.