It's still a work in progress, but I would say the Golden Lion Tamarin Conservation Program has been a success. Without these extreme efforts, golden lion tamarins might be extinct in the wild.
For a selection of projects on a wide variety of species in Scotland, you could look at the SNH Species Action Framework Handbook. Conservation evidence is another great resource.
I am not 100% sure what type of conservation project you are looking for, but there are many successful marine protected areas conservation projects around the world from tropical to temperate regions. I attached a few examples. Have a look at the references of Gallacher et al 2016 for many other successful MPA conservation projects.
I think that you should take a broad view of your own question. That is, whether or not a golden lion tamarin project (or any other project focused on a particular species) has been successful, the key point is getting natural systems set aside, and protected as completely as possible. That is, even if the project "fails," and the flagship species goes extinct, in the big picture, probably the protection of the natural system is more important, and the project may have been successful in that sense. Just a (rather radical) opinion...
In the island of Gran Canaria (Canary Islands, Spain) some conservation projects are being developed. One of them involves a reintroduction of a canarian endemic pigeon (Columba junoniae) through the regeneration of the laurel forest, its main habitat. After several years, around 140 pigeons were set free and half million trees were planted. Also, at least 30 individuals were born in freedom. So, in my opinion this is a conservation project that is being very successful.
More information is available in this web: http://liferabiche.com