All ukulele resonators I've seen or owned are single cone, with a biscuit (it acts as a saddle). That means the strings press on the biscuit (a wooden bar) and that vibrates a spun aluminum cone, which creates the sound (a bit like a speaker cone). Some sound is bounced around inside and escapes through any soundholes. The effect is a metallic and bright. An all-metal body tends to emphasize this and add some sustain.
The cones are thin and - if you disassemble the uke - easily dented or bent. If you don't, then they're fairly easy to maintain. Don't let your cat or dog sit on the cone if you remove it (voice of experience...).
There's a cover plate over the cone, mostly to protect it, but it can also muffle the output a bit if it doesn't have a lot of holes or space for the sound to escape. You can often remove or alter it (replace it with one that has bigger holes) to get more sound. Just be careful not to damage the cone.
Some care has to be taken when changing strings because the biscuits are usually held on by pressure, not glued or otherwise fixed in place. Some cones have circular areas on top that allow the biscuit to be moved around quite a bit, so use a marker to note where it sits first, so you can return it to that location after restringing.
You play them the same way you play any uke or guitar (unless it's one designed for steel slide as some guitar resos are).
The sound can also be altered by changing the biscuit. A lot of them are rosewood, some are maple, some are two-part. Using ebony can really brighten up the tone. I want one myself to experiment with other biscuit materials like Tusq and brass.
Only problem I've ever see with uke resos is that the nylon strings are anchored in a metal tail or cover and there's pressure on the intersection. This creates a potential breakage point. To avoid this, get some plastic wrap (the shrink wrap used for narrow-gauge electrical wire is perfect) and slip a small section on the end of each string so the metal presses on it, not the bare string.
it may be possible to use metal strings on a reso uke, but I haven't tried it myself.
See my review and some pics:
www.ianchadwick.com/ukuleles/republic.htm
The volume and quality of the sound depends on the tension on the biscuit. Which suggests that a tenor is the best size because its strings have the most tension. As the owner of a tenor reso guitar, I can attest that four strings son't provide the same depth and richness that six do because they can't put the same pressure on the biscuit. I personally don't think a concert-scale reso uke is as good sounding as a tenor. Same problem: not enough pressure. (Curiously, I think the opposite happens with a banjo uke: a tenor applies a bit too much pressure to the saddle>head, and unless the head is large (10") it sounds too brash: a concert or baritone banjo uke sounds better).
I've been trying to buy a Kala reso myself, for six months but can't find one - and the new Kalas are not expected in Canada until at least August. Maybe longer. *sigh*