Well honestly it's subjective, I personally prefer to buy add-ons for a current game I love and want to pump a few more hours into (20 hours for my part , I was basically paying 1euro/hour which is quite reasonable

) than just move on and buy another game which would probably be more expensive ! (I never buy secondhand and I don't buy cheap games cause cheap games are old and not up to date)
You can't actually measure anything other than the questline in this DLC . Why ? Because some additions stick with you for the rest of the game! I know I'm going to use the crossbow during the rest of the game and future DLCs with my assassin ! And starting a new character to be a vampire lord is immensely rich roleplay-wise (Having to change your habits because you're a vampire , sneaking up on sleeping villagers when they are asleep , transforminf into your original form when you face multiple ennemies, etc). Your new followers and pets will accompany you through the main game's questlines ! Having Arvak when you are overencumbered is also really neat when you'll be carrying too much loot !
And I know this is completely biased, if you have the game , finished the main quest and guilds and stopped there, I'd understand your choice not to buy the add-on. But for an addict like me, who has been looking up pages on the elder scrolls wiki to learn more about the dwemer and other races for the lore (no spoilers even though most of you haters are determined not to get it) , I managed to appreciate every bit of it, reading the new books as I went, crafting my Crossbow as I imagine my future stealth kills for the Dark Brotherhood Contracts ... It's just like Call of Duty, hardcoe players will love every bit of their new maps and it's not faire to compare them to sheep, pigeons or any other animal .
The more you whine, the more you hate the DLC . I have played before reading all of those nasty little threads; aftermath : I am satisfied with my experience and am ready for future Skyrim DLC . I'd okay with spending 20 euros for a game that has prevented me from buying new ones due to its lifespan and replayability !
You didn't like it ? Fine! you are entitled to your opinion !
PS : One thing that annoyed me was the locale marketing or whatever you call it . In France,only one option is available : the French version ! People who bought Skyrim proper in English (with or withouth subtitles)? Screwed ! I had to create a British account just to get Dawnguard in English ... Well I did save 3 euros thanks to the conversion from GBP to EUR...

is 17 euros still too expensive ?! haha