Sennheiser make great headphones for the price. I have a pair of the Sennheiser HD201 lying around and you'd struggle to find a better sounding headphone in that price range. Some of the higher end Sennheiser headphones sound beautiful.
Beats by Dr Dre are more of a brand/aesthetic, they're not even close to being worth the price for the sound they produce, compared to other headphones in that price range they are...underwhelming.
In a way they're a triumph of effective branding for a fairly mediocre product.