I was on Doryx (a type of doxycyclene) for 3 months. My acne was light-moderate, and Doryx made my skin flawless (even my back and chest!) in a very short time. The only problem is that as soon as I came off of it (typical treatment lasts only 3 months), I broke out HORRIBLY!! My skin was never that bad in my life. I developed cystic acne and my whole face hurt.
So, in a sense, yes doxycyclene works. But you have to be careful about stopping treatment--the acne could come back worse.
Now, the only thing that keeps my skin clear is B5 (10 grams daily--20 500mg capsules) Differin Gel, Retin-A micro, and proper exfoliation. I swear by the B5-- I don't have any pimples on my face now, the oiliness is gone, and my scars are diminishing every day.