Hi Anne, to expand on what friend Luke said above: service/access IS pretty good these days, but don't (blindly)

take my word for it ..take a look at the frequency/volume of complaints in the various (ADSL/itBust/Sentech) sub-forums and see where you get the least complaints. As to modems and contracts, sure you can go for a modem from someone here which should work out perfectly OK; afterall, there are no moving parts so it's like there's something to wear out! Only, as with a private sale IF something goes wrong you're on your own. Also, I know of two modems whose history can be
trusted.
But, as I'm in the biz I can offer you a used modem with a 6-month Sentech-backed warranty ..or you can go new anyway, the prices have recently come down. You can also keep (WELL!) clear of a 24 month (prison)sentence/lock-in by buying the modem outright, which would allow you to go for a 30-day notice contract.
And finally: the Fourways area is HUGELY hit-and-miss on account of it being served by the
WORST 
tower in the JHB area. I've got sites (roughly) south, east and west (and even out way out in the boonies NE) of it that, one way or another, have required using an external HGA
* to get access. That said, once done they work out great ..and I know this as, aside from having to redo one site by way of moving them up to a bigger/stronger HGA, I haven't had any complaints (and people DO complain when they pay for things that end up not working!)
* HGA = high gain antenna