Hugh - I echo your sentiments of visiting other camps to and from Addo...makes for a great experience allround.
We have been there in Jun'11 doing a circle route i.e. Wildernis, Addo, Mountain Zebra NP, Camdeboo and back via Outshoorn. In Jun'12 we did a Wildernis, Addo, Tsitstikamma route - both from Cape Town. Nice easy drive with great variety.
The personnel at Addo are brilliant people and most helpful. Addo can be accessed via the southern part i.e. N2 at Matyholweni camp and drive to the northern area to the Main camp, where we stayed. There is also a nice overnight hide/tented camp called Spekboom where one can stay and close by a picnic area with facilities for body breaks etc. It got a sighting board and there is also one at the main camp. Be careful of the black rhinos - they can be territorial and might surprise you...
The ellies are a lot calmer as those in Kruger. We had some great lion, hyena, jackal, eland, buffs, rhino and Red hartebeest sightings. Kudus and warthogs are like the impala is in Kruger. Lots of great birdies all round and the small animals in the fields.
One can do a drive through the main park twice a day, so more opportunities to spot the allusive ones. I cannot comment on the greater Addo region i.e. Kabouga, Zuurberg, Darlington sections. But definitely doable to drive from the main camp to the Woody Cape section - at the seaside and back for the day.
Hope the info help a bit.