Undermount sinks, sinks in general in my opinion add no substantial value to your home. If I was buying a house, I'd prefer to go with the quality of the structure of the house and not be too caught up in the fittings/sinks etc of the house. Most people will be looking at the size of the house, the functionality of the house, the quality/condition as well as the location and bang for money along with other things. However I do believe having stone/marble/granite bench tops will add value to your home but remember you won't necessarily get your money back from it when you sell.
I love undermount sinks and always will. I hate the drainage grill and love the idea of being able to add/remove the accessory when needed.
A lovely, well established garden can add more value to your home than the accessories/fittings you get fitted.