I used Manhattan Mini Storage this past summer when I moved from Brooklyn to the Upper West Side. There was a gap between when I had to vacate and when I could move into my new place so I stored my belongings at this place.
They have locations all around the city and are very professional. I would suggest checking out their website and not just showing up at one of the faciliities. For one, prices for the same size room vary by the location in the city. Therefore, if you don't care which of their facilities to use, you can save a lot of money.
I only stored for a month, but I was still able to get a truck to move everything. I got the truck for 4 hours for free. Definitely something to ask about.
Finally, the one area they do rip you off on is accessories. I would not buy boxes, packing tape, etc. there. Bring your own lock! (or you'll end up spending $14 for a $4 lock there).