Depends on your viewing distance and your eyesight. Also, there's this thing about 4:4:4 chroma subsampling, which affects readability of colored text, which is usually (but not always) a problem with TV sets. HDTVs usually have higher input lag, too, but that only matters to gamers, especially FPS gamers.
But if you currently have a 19-24" monitor with 1920x1080 resolution, then the pixels will be basically two times wider and higher on a 40" HDTV.
I'd say 32" is usually the borderline size to use HDTV as a monitor from a normal viewing distance of 40-60cm (~2ft). Bigger than that, you either have to be blind as a bat, or increase the viewing distance.
You'll just have to try it out, if it's good for you or not.