Getting it on the road with a tune-up, grease, and new tires is probably almost always worth it. Investing large sums of money in component upgrades won't be worth it on low-end bikes.

Pics will be for anybody here to evaluate where the bike falls on the "value" continuum. Things to watch for will be the tubing material, the dropouts (stamped or forged), and the components and wheels (steel or alloy). The closer the bike is to low-end, the more you will want to invest in the basics-- a tune-up, grease, and new tires-- and avoid the fancy upgrades.