Its not worth the upgrade over the f2.8 version unless you really need the f1.8. Other than being soft wide open at f1.8 (no surprises here), it exhibits all the characteristics a good prime has (sharp when stopped down, good colours/contrast, free from distortion and flare is well controled).
I sold my f2.8 for the f1.8 .... can't remember how much it cost but should be in the region of 800 (I think). Demand for this lens is not high so I doubt the price would have changed much since. A second hand should command 550-600.