I've got an MPK25 (soon upgrading to a 61 / 88) and use it pretty much as you have described, for working out melodies and the like. It's much easier than using your computer keyboard or piano roll. I only wish I had bought a bigger one to begin with as with only 25 keys you soon run out of space for two handed stuff.
As for the feel of the keys, they have a pleasant enough feel about them and are velocity sensitive with aftertouch, which is great for mapping to filter cutoffs and the like for your soft synth of choice. And as for the pads, I bought a simple insert kit for under £5 on eBay and it took me about 15 minutes to open up my MPK and install them. Makes a massive difference.
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