outdoorxplorer,
The only problem I encountered was when I wanted to switch back and forth between 3G and GPRS (to save battery life).

Once 3G is activated on a lower-capacity SIM card, it could lose the ability to revert to GPRS. A call to CS confirmed this as a known issue and I was offered two options:
(1) pay Maxis $25 to upgrade to a 3G SIM card which would give me the freedom to switch between 3G or GPRS
(2) deactivate the 3G service

This problem might not affect ALL non-3G SIM cards (depends on your luck) and in your case-after activating 3G-you might not want to use GPRS ever again. So, nothing to worry about.

This is why I am waiting for Maxis to implement OCS for prepaid users. Supposed to be by the end of this year but not sure if or when it will actually happen, knowing how Maxis likes to suck every last sen from us.

Aiiyah...you know, I know-lah *money money*

Right now, whenever a user has a credit deduction dispute with Maxis like ViRaViRa, how is he going to prove his case? With OCS, at least we have some black-and-white proof of our usage.