I can't tell you how annoying it is when you pass a driver on that drs straight, they start kersing and kersing and kersing to re-pass you, block you for the rest of that lap, and then pit on that lap so you can't even get drs to make up for it...
Oh yes, that's so annoying indeed. However that's part of the race, deciding on the spot to use that boost needed to go in front again (and possibly more boost if you fail to shake that car before the next DRS or if it's using more KERS to get back into DRS) or risk to stay behind.