It's hard to track down specific things from that wireheading.com site, but this seems to be a good overview. Of particular note are a couple of excerpts:

The results of these experiments indicate that reinforcing brain stimulation may have two distinct effects: (a) it activates pathways related to natural drives, and (b) it stimulates reinforcement pathways normally activated by natural rewards. The empirical observations seem to contradict classic "drive-reduction" theories of reinforcement (reinforcement appears to be associated with increased drive in the EBS paradigm). However, it is not difficult to construct a plausible alternate hypothesis: Animals may self-stimulate because the stimulation provides the experience of an intense drive that is instantly reduced due to the concurrent activation of related reward neurons. This interpretation accounts neatly for many of the apparent paradoxes we have already encountered. Priming is necessary, according to this interpretation, because EBS reinforcement not only activates reward pathways but also provides the reason why that should be pleasurable (Deutsch, 1976). (This also accounts for rapid extinction, as well as the decreased efficacy of intermittent reinforcement.) The hypothesis assumes that the reinforcing properties of EBS are determined by the degree of activation of related motivational systems. It therefore accounts readily for the observed interactions between the reinforcing properties of a stimulus and various experimental conditions that affect related primary drives such as hunger. When there is little endogenous activity, for instance immediately after a meal, the stimulation elicits only a small amount of drive-related activity. Concurrent activation of related reward circuits therefore can produce only a small reinforcement effect. When hunger-related neural pathways are already active because of deprivation, the same stimulation elicits more drive and hence more reinforcement. Indeed, it may arouse the drive system sufficiently to elicit consumatory behavior that further potentiates the reinforcing effects of the electrical stimulation.

...

It is interesting to note that while the animal literature suggests that brain stimulation has positive, reinforcing effects, the human literature indicates that relief of anxiety, depression and other unpleasant affective conditions may be the most common "reward" of electrical brain stimulation in humans. Patients with electrodes in the septum, thalamus, and periventricular gray of the midbrain often express euphoria because the stimulation seems to reduce existing negative affective reactions (even intractable pain appears to loose its affective impact). However, many psychiatrists caution that this may not reflect an activation of a basic reward mechanism (Delgado, 1976; Heath et al., 1968). Relief from chronic anxiety has been reported during and even long after stimulation of frontal cortex. Again, the experiential response appears to be relief rather than reward per se (Crow&Cooper, 1972).

In general, it seems as though electrical brain stimulation isn't quite as effective at producing bliss as one might wish (or fear).