'Nice medium-sized zoo' sums it up pretty well. There's been a lot of recent development, and from my travelling companions' reactions (who visited previously) there has been a marked improvement in the last few years. It's far from perfect, but it has a nice 'classic city zoo' parkland setting, few poor exhibits, a distinctive eye for design, an animal collection that balances the classic zoo animals with some nice rarities - very pleasing all around. It would be quite a nice zoo to have as your 'local' - easily 'doable' in half a day but equally easily able to fill a day perfectly well.
Highlights-wise - good caprine collection, huge Saltwater Crocodile, striking giraffe house, Arnold's Giant Tortoise and New Guinea Tree Monitor, the slightly bizarre but not unpleasing Prof. Brandes House, sousliks, the 'underground' zoo, the wonderful little 'Arctic' complex with foxes, fish and birds - and above all the run of little European bird aviaries, which are an absolute treasure!
And the little incursion into Germany enabled us to sate our zoo trip Currywurst urges! (as Osnabrueck had done on the Dutch trip earlier in the year!)