I can link you to an excellent article1, but unfortunately it is in dutch. I shall try to translate parts, and provide the linked articles too.
The article starts with some history and explanation. It seems that the makers of the TENS apartus claim it has some evidence. In 1950 it was banned by the FDA (the provided link is in dutch also).
A new theory2 about how pain works was introduced and claimed to be the working of TENS, but this was never proven at this point.
As of the proof, the point is made that it's not easily possible to perform a double blind test, as the electrical stimulation is felt. One can do meta-studies about the studies that are published (PubMed etc), and these are laid out in the artcicle (making it a meta-meta article). One of the good studies (score of '3', if that says anything to you) concludes that there is not much difference3
There are several other studies mentioned which are either concluding that there is no effect, or do find an effect, but have a low quality.
As a final thought, one of the referenced books' writer (who was first claimed to point to accupuncture as a valid scientific treatment) responded to the article. In this part he references some different studies. Using the words "TENS" and "clinical trial" and "human" he finds 210 'effectstudies' which would be the same word translated I guess. These studies are measured using "systematic reviews".
He references a studie that concludes there is short term diminishing of pain4, some research that says more research is to be done5. He cites some other research, but this rings some warning bells in my head: (rough translation, sorry)
TENS can, when used informed and guided correclty, lead to an active role of the patient, lowering of cost and improvement of functioning
More of the references linked to this part of the article are found under "more references"
This article of the dutch skeptical society leads me to believe that although there is some research in favor of TENS, that just as with accupunture, the evidence leads one to think it is more of an alternative medicine, and not real Science Based Medicine.
On this point I can link you to an article 6 which does a systematic review of excisting research. The results are that the quality on a scale of 1 to 5, there was only 1 that got a good grade, with a typical 2.1 average. The conclusion is that there is no evidence of pain-relief with TENS in labour.
: Melzack R, Wall PD, Pain mechanisms: a new theory. Science 150 (1965), p.971-
: Aker PD, Gross AR, Goldsmith CH, Peloso P, Conservative management of mechanical neck pain: systematic overview and meta-analysis. BMJ 313 (1996), p.1291-1296.
: Osiri M, Welch V, Brosseau L et al. Transcutaneous electrical nerve stimulation for knee osteoarthritis (Cochrane review). Cochrane Library, Issue 4, 2000.
: Carroll D, Moore RA, McQuay HJ et al. Transcutaneous electrical nerve stimulation for chronic pain (Cochrane review). Cochrane Library, Issue 4, 2001.
: Caroll D et al, Transcutaneous electrical nerve stimulation in labour pain: a systematic review. Br J Obstet Gynaecol 104 (1997), p.169-175.
Scherder E. Peripheral nerve stimulation in Alzheimer's disease (thesis). Vrije Universiteit, Amsterdam, 1995.
Jongste MJL de. Neurostimulation as an adjunct therapy for patients with intractable angina pectoris (thesis). Rijksuniversiteit Groningen, 1994.
Fishbein DA, Chabal C, Abbott A et al. Transcutaneous electrical nerve stimulation: treatment outcome in long term users. Clin J Pain, 1996, 12: 201-14.