Flat pack furniture is great; you don't have to worry if it will fit through the door, you can easily move pieces around to the right room to assemble, and it's usually cheep but functional.
The pain, both figuratively and physically, comes when it takes six, yes six, hours to assemble a single item of furniture! No I'm not incompetent! It was a big sideboard and one of the pieces had been drilled the wrong way around necessitating a 20 minute detour from the instructions while I disassembled instead of assembled the base unit.
I put it together Sunday afternoon and my hands, legs and back still haven't recovered from the experience. The palms of my hands are actually bruised from gripping the screwdriver.
Maybe next time it would be easier just to buy ready assembled furniture after all!