I think you should always try the demo software for games before you purchase the full version. Often gamers just like to pick up the newest titles, but to avoid disappointment the free trials are a good way to go.
I have found that with some games, you get bored of them after a week. Most free trials of software last for around 2 weeks and up to 30 days. This is plenty of time to see whether or not the game is worth purchasing.
Also, most MMORPGs provide a free trial. This is excellent as the online community is always changing and you need to know if you'll like the crowd once you get in game.