There Will Be Blood is decent, maybe even good. I'd say it's the best of the PTA films I've seen so far (that's not necessarily a great compliment). For all of its flaws (namely bad judgment in editing, serious overacting - especially Dano but also DDL in the last sequence, annoying use of Greenwood's "music", uninspired outdoors cinematography - indoors photo was great - and PTA' overall inability to connect with the audience on an emotional level), the film sort of works IMO.
Clearly, it's no Citizen Kane and PTA continues to channel Scorsese, but it tells a story and PTA seems to have put aside some of the manierism that was crippling his previous films (Magnolia and Punch Drunk Love being the worst examples IMO) to focus on the story and try to deepen character exploration. The humour in TWBB is also more subtle than in PTA's previous works and is conveyed well in DDL's expectedly great performance.
It's not a great film but it's worth seeing IMO. But without DDL, I don't think it would be. I still don't think that PTA has much talent for film-making.