Abuko is a wonderfully intact piece of tropical forest, amongst the otherwise heavily populated Serrekunda area. Some reviewers seem to be under the impression this a zoo, it's not! There is a small animal orphanage at one end of the reserve but its a desperately sad place and not a reason to visit.
I enjoyed wandering the narrow paths seeing birds, lizards and monkeys (you will see them if you walk slowly and quietly). The Darwin field centre is an excellent vantage point to see lots of wading birds and birds of prey as well as Nile Crocodiles.
The guides are knowledgeable and well worth taking along on an initial visit. I agree with the previous reviewer who advised taking water with you, it's quite a long way to do the full reserve circuit and the heat is sapping. The reserve doesn't get much funding so facilities are basic, but I loved it for being so quiet without lots of other people around.
I would recommend Abuko more to the serious nature lover who will get more from it than someone who expects to be entertained and see an animal every 2 minutes. For me, taking time was rewarded by getting to see amazing species like the Giant Kingfisher close up. I enjoyed it so much I actually went twice during my week in The Gambia.