I had a EZ Flash 3-in-1 expansion pack for a while with a SCDSOne as well. Slot 2 expansions are completely necessary for using the Opera DS Browser unless you have a DSi. The browser itself is crap. Although the DSi version is considerably faster.
Here are a few things I myself have used my expansion for-
- GBA compatibility (i love this feature)
- Ram expansion for SnemulDS (this helps games over 4MB to not stutter as badly)
- DS web browser (I hate this thing, but sometimes I need to check a site on the fly. there are third party browsers that don't require the expansion as well though)
I'm likely missing some features other people use. I hear Quake requires the expansion to get anywhere close to decent speeds, or maybe it needs it to boot at all. Never used a DS Quake.
At any rate, here are a few popular expansion choices-
- EZ Flash 3-in-1 ($20 at Dealextreme free shipping. This one includes rumble support)
- Ewin expansion (found it for about $13 at one site, similar to the 3-in-1, no rumble)
- M3 Real expansion (also can be found for $20, no rumble)
- Opera expansion (can only be used as memory expansion, no GBA support or rumble, needs to be bought with the DS Browser) I do NOT recommend this since the features are rather pathetic.
Basically the best feature set is the EZ flash. I myself own one and have used it for over a year. I can say it was great while it lasted. The issue is that the internal battery ends up dying after less that a year of use. You can still use it for memory expansion or rumble support after that, but your GBA games will no longer save after the battery dies. You can either buy a new one or try to replace the battery. I tried to replace the battery and somehow the EZ Flash shorted out my DS Lite (apparently the card was already bent inside and caused some sort of electrical screwup when I tried to put the card back together). It's not really worth trying to fix the card IMO, but it's a decent card while it works properly. I'm not sure about the new 3-in-1+ revisions or whether the batteries work better.