I bought a Korg nano pad 2, mainly because it was cheap (~£35) and I love it. For the price the quality is excellent and has a relatively small footprint for my small office studio. It is 2 x 8 rather than 4x4 which can be a bit confusing at first but very easy to get used to. I mainly use it with Kong and RDK2.0 and have set up custom 'scenes' to get the most out of each device (and make it more comfortable) which is easy and once you've done it it's done.
I've not used it with a Dr OctoRex but I have set up various loops using a Nurse Rex in Kong which is a lot of fun.