<p>I'm looking at developing a device which will need to support Ethernet over USB (hosted in Linux, XP, and Vista). As I understand it, Vista and Linux support the industry standard USB CDC. However, in classic Windows style, XP only supports it's own Remote NDIS. So, now I'm thinking of just bowing down and doing it over RNDIS, as opposed to rolling my own CDC driver for XP.</p>

<p>I've been reading some older documentation that says even XP is pretty buggy with NDIS (suprise!). Does anyone have experience with XP's RNDIS drivers? Are they safe for product development? Any insight would be much appreciated.</p>

The filesystem backend is only worth looking at if you're not going to use a database for any other part of your system. If you are using a database then the filesystem backend has nothing to recommend it.
The memcache backend is much quicker than the database backend, but you run the risk of a session being purged and some of your session data being lost.
If you're a really, really high traffic website and code carefully so you can cope with losing a session then use memcache. If you're not using a database use the file system cache, but the default database backend is the best, safest and simplest option in almost all cases.