Actually I bought Dawnguard the other day and despite reading all the negative comments on the net I have to say I really enjoyed it. All the mods I'm currently using work fine and quite of few of them are "Dawnguard required" ones anyway.
Good things: Great companion with an interesting backstory and decent voice acting. Crossbows (love crossbows
) DG Makes vampires and werewolves better than the vanilla game (still not great but definately improved). Great environments, Forgotten Vale is incredible to see, some of the caves and dungeons are very artistically done. Good amount of developed lore concerning the Falmer. Some nice new armour.
Bad things: Questline is almost identical regardless of which of the two factions you choose. Ending is something of an anticlimax (kind of the feeling the player got at the end of the civil war questline). Spawning vampire attacks at night in cities can end up killing NPC's. (Solved this with the When Vampires Attack mod, which makes civillians run indoors).
Vampire Lord form is annoying since you can't interact with loot and sometimes you have to revert to human form to fit through certain passageways. Very linear questline with a complete lack of choice (typical Bethesda state of affairs
) Way too many Elder Scrolls, a TES game should never have more than one, with DG it seems like everyone seems to have one.
In all honesty theres nothing really bad about DG and I think its worthwhile if you want more Skyrim related stuff. I certainly thought it was worth the price.