They are the single most useful method of medium-or-larger distance transport I've yet encountered. They're nearly completely configurable as to locations, they can work intra-dimension as well as inter-dimension... They fix every single problem I've ever had with a transport system, and they don't require power
Which is also why mystcraft is the single biggest OP mod that I just love and want to use.
I love the dimension/world creation.
The teleport system is just badly too strong.
I look forward to Veo's linking tweaks being ported to 1.7.10, so that mystcraft portals can be restricted to spawn only, or some other restriction on them.
As much as horses/donkeys made minecarts, boat roads, nether rails, etc, all obsolete, mystcraft portals make horses (and everything else) almost obsolete.
Horses still win for the first trip to a new location.
Portals win for everything else.
Minecarts, waterways, etc, are all useless once mystcraft shows up.
And ... Ender TNT that will teleport to, and explode at, arbitrary locations in arbitrary dimensions? It's slightly less powerful than mystcraft portals, but it's still triggering my "balance sense" .