My two nanos (both 16Gb 5th gen) have been quite reliable. Video life now is about 4 - 4.5 hours, down from 5.5 after two years; this is definitely long enough for any movie, probably two unless it's "Avatar" and "Titanic" back-to-back, and the 5th gen's built-in speaker allows you to share the fun. I've been saved multiple times on long car trips by having a couple of children's programs "in reserve" and letting my daughter hold it.
Music playback is easily the better part of a day on continuous playback.
I can't say definitively about radio battery life, as I seldom listen to radio. I would assume, though, that since the backlight turns off during radio play (as per the normal backlight settings), many hours of radio enjoyment can be had. NOTE: The radio function uses headphones as the antenna and thus will not work through the built-in speaker. This and the click wheel, which some view as uncomfortably small, are IMHO the only "flaws" exhibited by this generation.
The screen can be somewhat prone to scratches. Most cases for the 5th gen come with a screen film, but if you buy a case that protects the rest of the machine, it will prevent most nicks and scrapes on the screen itself by preventing surfaces from contacting the glass during drops. I highly recommend the Scosche Kickback n5, with a combination of clear plastic back and rubber edge bumpers, and a built-in kickstand for propping it on a table.
In summation: The 5th gen is easily the most feature-rich of the entire nano line (video camera, anyone?), and I can think of no better replacement for your ailing classic, beside another classic of course. The trick nowadays is finding one in decent condition without paying out the wazoo.