I live just up the road from there.
Been many, many times.
There are two parts to the reserve. There is the captive area which does contain a large selection of different birds from all over the world. Some of these are in reasonably natural looking areas. But they are wing clipped. Some folk abhor that, others accept that it's a necessary evil. If it wasn't for WWT then the nene would be extinct, and the spoonbilled sandpiper would also be close to extinct. Good or bad? You decide. Lots of stuff will be in breeding plumage at present.
They are very much into the tourist market. Lots of play areas for kids and interactive displays. Not for me, but you may think differently. They have built a large "netted" area where unclipped birds can fly free. Crowned cranes, avocets etc., were in there last time I visited. It's becoming a bit like Alton Towers with feathers, but that brings kids, and they bring parents...and their money!!!!! Nice, but pricey cafe.
They also have a beaver reserve, but they are usually asleep in daytime. Otters too.
Birds are relatively used to humans so you can get close. Take or buy a bag of food. Lots of stuff will eat out of your hand, or at least come very close to you.
The other part to the reseve is the free area. This is outside the big fenced area of the reserve. There are several hides dotted about, including one very nice new one. The main attraction they overlook is the mere - as in Martin Mere. A big wetland area that in winter is full of migrating swans, geese, ducks and other wildfowl. Totally free flying. They will all have left now to return to their winter breeding grounds. The water level here is managed to account for different seasons. In summer it is lower to encourage the waders - egrets, avocets etc. Out here you really need to be a full on twitcher. There is lots to see. Tawny owls, swallows nests and all the usual lbjs, but they are seldom sat there waitng for you. Patience is a virtue - and I don't have it! Lots of one time visitors so check at the information desk before you wander out. In Focus have a shop in the wild area; that's a good place to ask what's around and he has good scopes set up looking out onto the mere.
My subs to WWT expire tomorrow and I haven't renewed. I've been many times and not being a twitcher "been there, done that". But if it's your first time then you should have a good day. It's interesting. I think it's half term round here - not too sure because it always seems to be half term somewhere, so you may be plagued by grandparents and their offspring's offspring.
Far from the best shot, but shows how close you can get
Picture by Frank Yates by
Frank Yates2010, on Flickr