I agree with Joey on every single count. Here's my two cents on these machines:
1. Seated Leg Extension
Many guys overload this machine and kick up the weights, arching their back and slamming back against the seat. Lower weight and a controlled, fluid move should make this move much safer. I avoid it, anyway, in favor of a dozen other quad exercises I do off of the machines.
2. Seated Military Press
The pathway on many of these machines is very unnatural. Nevertheless, a lot of guys do shoulder presses with dumbbells using the same pathway.
3. Seated Lat Pull-Down (Behind the Neck)
"Craning" the neck during any exercise can lead to cervical disk problems. The weight load exacerbates this problem.
4. Seated Pec Deck
A lot of guys also "crane" their necks on this machine, trying to assist with their shoulder muscles in an unnatural path.
5. Seated Hip Abductor Machine
In a controlled movement with a limited range of motion, I don't believe these machines are all that bad.
6. Seated Rotation Machine
A horrible device.
7. Seated Leg Press
Joey said it all.
8. Squats Using Smith Machine
I know tons of guys who complain incessantly about knee and lower back pain, but load this machine up like they're in one of those strongest man competitions.
9. Roman Chair Back Extension
Okay if you're fourteen, lean and limber with no pre-existing back injury.
10. Roman Chair Sit-Up
This isn't the worst machine on the list, but too many guys seem to power through this exercise, working the wrong muscles and allowing momentum to take them through the move.