100% one of the best things you can do to a neglected car. Rub your hand over the paint and you will feel/hear lots of rough patches (amplifies it if you put your hand in a plastic bag), particularly low down by the wheel arches. A clay bar will lift all of this off and make the paint smooth as glass.
Tar removers and iron X are good products for helping the decontamination, but clay bar is always the way for me. Gets rid of other marks, oxidation and sap, as well as tar and iron. Those products just help remove the heavy stuff to reduce the chances of marring, but I tend to think it is inevitable you will leave some marks behind..even with the lightest clay bars (buy a good brand one, cheap ones are even worse...) and plenty of lubrication (either a quick detailer or just soapy water) I still get light marring and marks. I therefore only do a clay If I am prepared to do a light polish afterwards.
Maybe others will disagree, and some will say it depends how closely you are looking at the paintwork, but thats just my experience....I am a little on the extreme detail side...!
