I don't use it myself, but there was a useful discussion about it at work recently.
If you can't get broadband - or you are at the limit of ADSL range - it can usefully accelerate some content.
It will accelerate email and most HTTP traffic, including both compressed and uncompressed JPEG, PNG and GIF files.
Note that it will not accelerate browsing over HTTPS (secure websites), file downloads or uploads such as QuickTime files, MP3, AVI, MPEG, exes or streaming media content (over FTP or file sharing programs).
And it has no effect on VPN connections, obviously.
If most of your internet connection usage falls outside the above, and you can't spend the money on broadband / faster broadband, then go for it!
SD