Hey Robone,
Until a week ago I was also doing the '24/7 very nice, even if speeds are slow' bit. Then I actually started to track my speeds - and the following is my personal opinion.
When to take Sentech: If you absolutely need 24/7 connection, thus you are at home during the day, and you need to have large amounts of data coming through. Sorry, let's change that to - not be capped as at recent speeds there are no large amounts of data coming through. Just be prepared that some days it will be so slow that pages will time out.
If you have ISDN and Callmore and only use your connection after hours, keep it. Especially if it is a 128K ISDN connection. Your ISDN speeds will be superior (even to the 256k service).
If you have ADSL, rather get a second account if you find you are hitting the cap. At least you will get throughput. Yes, I know this is sacrilege, but after speaking to an ADSL user (and he sparked my speed research) it seems to be a logical option.
Dial-up connection (analog, not ISDN). Difficult one this - if you are in a bad area, then yes, MyWireless may be the way to go. If not continue (R7 callmore) or consider ISDN.
I used to run on R7 callmore and ISDN (128k). After seven I was connected continuously until seven the next morning, and continually over the weekend. Total telephone bill was between R1000 and R1200 (this includes all costs, including line rental, Callmore etc). It has now dropped to about R500, so with the myWireless charge works out to be about the same.
This is personal experience - it may very well vary for other people. But if you are expecting similar speeds / throughput to an ISDN connection chances are extremely good you ain't gonna get it.