I used to have an ES. Its very much like a electronic Spotmatic, with aperture priority mode. The metering doesn't work on manual mode (if I remember correctly), and the manual speeds are a bit limited (1/60-1/1000). It may also be difficult to repair if some electronic components fail. Other than that, its a nice camera!
I had one, but traded it in for a Chinon CE 3 in the 80s. The Chinon has full range of shutter speeds, works in both auto and manual modes, and has a winder, (not very fast) while only a few of the ES bodies took the drive.
The Pentax I wish I could have had from new! I've got an ESII that I've used extensively. It may not have exposure lock, as has been mentioned, but it does have exposure compensation (don't forget to put it back to 1x, though!) Though I'm cynical of pretty much anything that professes to be automatic, I've always been very impressed by the ESII's abilities under a variety of lighting conditions. I like the fact that the 60th to 1000th speeds are independent of battery power - not a full range but enough to get you out of a hole if the worst should happen. I also like the extra position on the shutter speed dial for the viewfinder blinds, which I use a lot for tripod shots. I also have the earlier ES which lacks the blinds, D/A, shutter lock and indications of speeds to 8 secs even though it does them. Clever electronics/electromagnetic shutter release also means that it's not a battery gobbler as many early auto SLRs were. Go for it!