We did it and found Ardastra charming, but the tour was not that good. It did take us past a bunch of interesting historical buildings and locations (far too quickly), but then the tour guide apparently had run out of stuff to look at but had an hour to kill and he started pointing out all the gas stations on the island, all the fast food restaurants, etc. I kid you not. We also stopped briefly at Fort Charlotte, but not long enough to go inside. It was basically an opportunity to see the outside of the fort and a very nice view from the hillside.
Ardastra is small but nice. They did have a few unusual animals, including some Australian black swans, which we had not seen before, and quite a few tropical birds. The flamingos were kind of a hoot. They're just wandering around the grounds in the open, but when they do the show, the trainer rounds them up and herds them to the show area (as seen in BethC's photo), where they do a bit of "marching," and then people can pose with them and take pictures. It was fun; they don't take it super seriously, and the birds themselves are fun to watch.
I recommend just going there in a taxi (and then have Ardastra call you a taxi to get back). It's not far, and it shouldn't be more than $10-15 from the cruise terminal. You can walk, but it's probably 20-30 minutes in the heat. Or you can take a jitney most of the way there and then walk the last 5-10 minutes. If you don't take a cab, definitely get it marked on a map at the information center at the cruise terminal, or print out directions before you leave. It's kind of tucked in among some houses and it's not easy to locate. There may be signs on the street saying "this way to Ardastra" but if so we didn't notice them.
Have fun!