They are great if you are just starting out. If you can easily do more than 10 (and I hope you can), then you need to start moving up to bench presses--at a gym with barbells. The gym is for safety (it's a dangerous exercise), and barbells for better resistance (ie. no press machines).
If you can't afford a gym then you can try usind dumbells, not barbells, on a bench at home, but this isn't as effective.