Netscape's accelerator uses combination of compression, caching of already compressed content and filtering of unwanted content (adds, etc).
Some other accelerators are using host tables to speed up DNS lookups, some are simply proxy servers.
Some are combination of the above.
All methods speed up loading of webpages to some extent, some more some less.
Compression works on both static and dynamic content, caching methods are pretty useless with dynamic content. Proxies are generally trouble when it comes to authentication (online banking, etc), and Host tables are trouble too because of IP changes.
None will speed up mail, downloads, online gaming...
Netzero and netscape acceleration methods won't cause too many problems, while some others are nothing more than "scam".
My $0.02, stay with cable.