From what I have read so far it is plug and play and I plan on doing it hopefully in the next month.
The navi and non-navi units changed in 06, so if you have on 04-05 navi I would look for 04-05 non-navi. Or if you have 06+ I would look for 06+ only.
Also the clock screen above the center air vents is different between navi and non-navi cars. These are the same for all years though.
I installed a non-navi radio (from a 2006) in my navi tsx (2008) today. There is a black connector for the navi unit that is not used but everything else plugs right in. The clock screen above the center vents does not work, it shows a garbled mess. You can get the non-navi clock or just leave it unplugged like I did. The non-navi unit doesn't do voice commands so the voice command buttons on the steering wheel do nothing now and could probably be removed.
I plan to modify an aftermarket unit into the stock one so I won't bother replacing my clock screen since the aftermarket unit will have a clock. I also don't think I would be able to set the clock once I remove the stock radio controls. I am going to wait and see if the aftermarket unit with a steering wheel controller will let me use the voice command buttons, if not I will remove them.