Historical status

Does it increase or decrease the value of the units?

One thing it does? It makes it a pain in the A#SS to get any renovation that affects the exterior of the building such as replace windows, done. You need permission from everyone and their grandmother and you may even need to, in addition to paying for permits, hire an architect to get it all done.

It can make maintenance on the building more expensive and cumbersome.

Does it force the building to make improvements to the facade to be in keeping with the architecture of the time? We currently have doors that are more like the entrance to a Duane Reade than a building that is part of a historical district.

there is a responsibility..

Landmarks doesn't force you to make changes. If you do decide to alter or replace your doors, windows, etc., then you need approval for the change.

See http://www.nyc.gov/html/lpc/html/working_with/working_with.shtml