Pompeii is really interesting. Unfortunately, we did not climb Mt Vesuvius as it was raining too much and they closed it. Our guide was really good!
The bus does not come by often (at least in winter).
Actually, we finally went to Ephesus... beware, as Izmir is not the Turkish name for Ephesus (that's what I tought, from the documentation of our cruiseship..., and I met others who tought the same). Ephesus is really great, you must see it. As for Izmir, it looks nice, but we did not see it a lot..
We used Hop-on Hop-off in 4-5 cities in the last year, and I have to say that this one was a good one. It took us to the good spots quickly and was on time. Also, the audio system was working fine (not like in other hop on hop off we took).