Doing a bit of looking around online, I was intrigued by so-called "Freeline" skates (Wikipedia article appears a bit anaemic), which are very much like the old snakeboard, albeit without the connecting bar between the two plates. To propel yourself forward, rather than kicking off the floor like with skateboards and inline skates, you use your hips, legs, shoulders, arms and whatever you can to swing your body weight around and turn your feet so the skates follow a kind of S-curve.
It's called "non-holonomic motion" and I found it very difficult to understand via textual descriptions which were far superior to this one, so basically you just have to try it until you get the knack, which took me quite a while on the Freelines.
So I picked up a pair on eBay around Christmas (very expensive too, ended up around €110 delivered from the UK).
The initial hurdles
1. Standing on the skates without doing the splits. As you stand on the skates, the axis of movement is just off horizontal so the skates want to slide out to your left and right and you accidentally do Jean-Claude Van Damme sidesplits. Most people, including myself, can't really do that without our legs falling off and exploding, so you quickly learn to control the relative position of the skates with your inner thigh muscles.
2. Rolling across the room with losing your balance. This is presumably the same with any kind of skates/skateboard, but exacerbated by the fact that these behave in a slightly more unexpected way due to the wheels being angled slightly differently: there is a distinct left skate and right skate, so when they're side by side the wheels make a shallow V-shape. Getting used to this means rolling back and forth on flat ground by a railing or low wall so you can pull/push yourself and keep steady with your arms, until eventually you can coast a few metres unaided. Or get a friend to hold one hand and walk back and forth, but that's limited by their patience since it can take a long time.
3. Launching from stop without pushing against a wall. It's difficult to even stand still on the skates - a little bit like on a bicycle. Sometimes you see bicycle couriers with gearless bikes and no freewheeling clutch, stopped at a traffic light (yes, many bicycle couriers actually stop at lights :D) and very slowly inching forwards and backwards on the spot rather than putting their foot down. You can do a similar trick with these by angling your feet into a T-shape, but it takes practice, and actually pushing off from that position to get moving is even more difficult (I tried it about ten times and succeeded once, and awkwardly at that).
Normally, to push off moving from right to left, you put both skates in front of you, then knock the right skate over away from you and step onto it so your toes are also touching the floor. Then you put your weight on those toes and place your left foot on its skate, and push in that direction (without extending too far or your right foot will fall off the skate), pulling the right skate upright with your foot as some of your weight moves onto the left skate.
4. Propelling yourself! After all the other challenges, this one is pretty easy, to begin with anyway. Your feet will probably start to do this automatically in step 2, when you're just trying to roll without falling. Essentially, as one foot is moving in an upwards curve, pushing it forward will increase your speed, and similarly when it's on downwards curve.
Well, I only got them to replace my bike, so all the tricks people do with them (mostly pirouettes, riding on one foot and dropping/stomping the second skate, switching etc) are not really relevant. If I ever get supremely good at riding the things, maybe I'll experiment a bit.
Mostly, I want to find out if I can travel any kind of significant distance on them, somewhere between walking and cycling speed. A measly 8km/h for 4km (the distance between my house and DCU) would suit me just fine, but so far I have to stop riding after about 1km (about 10 minutes) because I get pains in the following places: the middle of the soles of my feet, my calf muscles, my instep. By then my thighs basically just run out of energy and I start moving more and more slowly and my form gets sloppy.
I don't know if this is poor general fitness (I'm a lazy, lazy man who likes sitting on his arse with a laptop or playing PS2 for hours on end), or specific muscle conditioning that needs to happen for everyone, or if I'm literally physically incapable of adapting to the skates for longer periods, or even if the skates are just not suited for that kind of travel, especially over the crap, laughable pavement conditions we have here in Dublin (stupid 1 inch deep, 3 inch wide drainage channels between every few houses, cracks and bumps everywhere, corrugated concrete driveways which make skates trundle and vibrate like a broken shopping cart).
That said, if you're going at a reasonable speed you can get over small cracks and drains by leaning slightly onto your back foot - that way the front skate can bounce past the obstacle and even if it gets caught and stops dead, you just hop off since they're not tied to your feet. If your front skate gets caught and most of your weight is on that foot, your balance can be thrown off badly.
And I've seen a couple of videos on Youtube which suggest that people can and do cover more significant distances on them - about 34km in this one.
One recent change that's working much better for me is focusing more on my back foot to push myself forward, rather than trying to drag myself with the front foot which causes it to slip out of position and seems to tire me out. Maybe it's just more efficient for the way I happen to distribute my weight on the skates. Who knows. Even if it turns out to be a complete failure, it'll be a fun failure!