I bought an EMS unit a couple of years ago, and I have to say that spending at least a bit of extra money will get you a LOT more usability. First, if you can get a unit that will allow you to set the rest periods (as well as number of repetitions and sets) that allow for Charlie’s max contraction settings. Most inexpensive EMS units I found have rest settings of only 30 seconds, not the 50 seconds needed.
In addition, I think the number of channels and pads is really important. If you have four channels with two pads each, you can get through your max contraction work twice as fast as a basic unit. That’s a big deal over the long term!
In other words, you get what you pay for, so consider paying a bit more. You don’t have to go as high as buying a Globus. I bought this model:
http://www.win-health.co.uk/neurotrac_sports_xl.html
It’s not bad for an entry level unit. The fact that it is a million times easier to use than a rudimentary unit means that you will likely use it much more often.
T