I owned and sold an OP-1. It is a beautiful piece of hardware and its analog controls and mashable buttons provide a more tactile experience than an iPad does.
That said, the sounds it ships with really leave something to be desired. My subjective opinion is that anyone who's played with a best in class software synth (i.e., equator2) is going to find themselves feeling pretty limited by the blippy tones that you get out of the box.
Sure, you can load up the sampler with tons of cool tones and op1.fun is a wonderful resource, but it's a lot of work to go curate a bunch of sounds to compensate for a lackluster palette.
I can already hear people saying that I'm missing the point and that the constraints are designed to inspire creativity, etc. I respect that point of view, but at the end of the day, my creative time is at a premium, and I'd like to use that time to create stuff that I'm proud to show friends and family.
Most of the stuff I produced with the OP-1 sounded like video game music or music for a kid's cartoon (i.e., Adventure Time). If that's your thing, then awesome! If not, I'd consider the core tonal possibilities before you make a big investment.