Well the giggy bank thing works out in some cases.. like if you occasionally use very little[1-2GB] and the sometimes need a lot[3+] but vary month to month on extremes. I can't remember where i put it.. but i worked out how the cost for having the saving can be good and bad. one exapmle i had was looking at before signing up for it was my last 1o months of adsl access, i added up all the 'lost' GB's i.e.
For ADSL384k
Dec2005 - Sept2006 = 10 months (i did 10 months since the maths will be easy
)
Amount GB Unused = 3.954GB (wow, 4GB
)
Number months used >95% = 3 (u know where u go into low saving mode to make it thru the month
)
Number months used < 80% = 3 (those few when i save up to 800MB)
Cost for 4GB closest ~3GB if bought would have cost R 189.00 (usable in 1 month only on old system)
Cost for 10 months saving option = R21x10 = R210 (3GB carry)
i.e. for R30 i have the convenience of spreading an extra 4GB of access that if bought(3GB) on just 1 month where u need to run over would just disappear at the end of month possibly not fully used. But you could argue that i just would stick to the limit.. i guess.. but this way I'd have more to play with. Also could argue 4GB wouldn't carry.. but then because you spreading it you can use it before it over fills the carry so you won't actually have 4GB stored at one time but still effectively use that amount.
Now thats with out the top up system where this system actually start looking good too cause if you top up, you can spend it maybe some this month and then the next etc which can be useful.
So yah.. thats why I like it
Edit:
Yes i know that means u pay for 3GB essentially.. thats why my comment one it being useful and not too. if you over a period do not save/use >3GB via the top up system you run at a loss. so for my one friend who always uses 90-95% of 3GB cap its not really gonna save him and works out more expensive.
Also have 3GB with 384K line.