There is a whole Yahoo! forum for endurance athletes, hosted by Hammer Nutrition, in which most users are triathletes, cycling enthusiasts and endurance runners (users of either Globus or Compex devices).
For endurance adaptation you have to increase the percentage of slow twitch fibers. The article, Functional and biochemical properties of chronically stimulated human skeletal muscle, Nuhr, Pette et al. 2003, European Journal of Applied Physiology, describes results showing increase in slow twitch fibers in humans, but the protocol was many hours of EMS per day. I’m, convinced you can obtain results with shorter sessions.
However, it seems from the endurance forum members mentioned above, that the most important advantage for them is owed to the use of the Active Recovery program. This program allows the athlete to recover from intense training much faster, and to be able to pack more training sessions within one weeks: also according to this video by a CSCS endurance coach. Two recent research articles seem to confirm experimentally that EMS active recovery works:
[ul]
[li]Effectiveness of active versus passive recovery strategies after futsal games, J Strength Cond Res, 2008.[/li][li]Effects of different recovery interventions on anaerobic performances following preseason soccer training, J Strength Cond Res, 2007.[/li][/ul]
The main muscle groups to target would be quads, calves and hams. The Yahoo forum has several resources on periodization for endurance athletes (see the links section in it).