Every national park in South Africa is compared to Kruger, and that's probably the right comparison. I'll state it flatly; Addo is interesting and enjoyable, but it's not Kruger. And if you're heading to Kruger, you can save yourself the time and money and skip Addo -- you won't be missing anything.
But if you're NOT visiting Kruger -- and we weren't on this trip -- Addo is a nice place to see wildlife and the main rest camp is a good place to stay. We had one of the premium rondavel rooms and I'd highly recommend reserving one if you can. As noted by other reviewers, the rondavel looks over a lighted waterhole where there are SOMETIMES animals. But keep in mind that the animals move around a lot, so it's unpredictable whether they'll be there the sametime you are! When we arrived at mid-day, the waterhole looked like wild kingdom; but after an hour, all the animals were gone and we never saw them there again! The rondavels are very pretty and comfortable, with a refrigerator and braai pit directly outside, and the full communal kitchen very close-by. Alas, the units do get a little buggy (don't leave your door open and lights on at night!), so they're probably not great if you're a bug-a-phobe (but then no accommodation at the camp would likely be good for bug-a-phobes).
Driving your own car around the park is a great low-cost, high-adventure strategy and, unless you're extremely unlucky, I think you'll see a large number of animals. Elephants, zebras, warthogs and kudo were everywhere for us, and we saw other animals as well (alas, no lions -- you'd have to get lucky to see lions at Addo). The camp offers several guided tours, but I think these are a waste of money (others will probably disagree).
The camp itself is very pleasant and compares favorably in amenities to those in Kruger. We didn't try the restaurant, but it looked fine, and the shop is well stocked and not too expensive (still, it's probably better to do your own braai, and stock up in town before arriving in the park). Try a marula ice pop from the store. Two days and one night is probably adequate for most, because you're likely to get "animal sighting fatigue" after a day or two! I'd recommend driving from Addo town directly to the rest camp (only a few kilometers away) and then, eventually, exiting the park from the south back to the highway and Port Elizabeth.
This review is the subjective opinion of a TripAdvisor member and not of TripAdvisor LLC.