I've done horizontal portals before. They aren't actually as bad as you might think, although you can't see where you're going through them (oddly you can see creatures/torches/plants but not blocks... period), so don't go jumping through any you don't trust. Typically you'd have both portals set up in the floor, and if you fall or jump into one you launch out of the other one with a bit of upward momentum, and a fair bit of sideways momentum (you have to actively try to have much chance of falling back into the portal you just shot out of). You could also set up both portals in ceilings three blocks above the floor so that people can walk under and jump up.
The only nasty part comes if you try to align the portals the other way, up-to-down/down-to-up. That one has a bad habit of dropping you straight back in when you try to jump up through it, and it can launch you in totally random directions at fairly high velocity as well last time I tried it.