We're using Wingate 5.2.2 on Windows 2000 Server and I've stumbled upon a serious issue. When users download files via FTP proxy with FlashGet, only a small fraction of session traffic is accounted (about 1-10%). I checked it from two different PC's. When the proxy is disabled and they browse FTP via NAT, everything seems to be fine. When I downloaded some files with Firefox via FTP proxy the traffic was also calculated correctly. So this is some issue with Flashget+Wingate, but it's very serious for those applications, where traffic accounting is vital.
I'll run a series of tests later with different download managers and Wingate 5.2.3. For the time being, I had to disable FTP proxy.