Well there are different smith machines out there. The one in my gym just has the rods that guides the built in attached 45lbs barbell, and the hooks that can secure the barbell incase they are too heavy or the users is fatigued by just turning the barbell. No pully system attatched to it so there is no "mechanical advantage" comming into play. I like this version of the smith machine since I've been working out by my self, on my schedule. I can lift heavy without a need for a spotter. No probs doing flat and incline benchpress. I'll admit though, it is easier to use the smith vs the regular free barbell since on the smith imbalances between left, right and core balance muscles for the benchpress is eliminated. But I can't complain. I'm lifting heavier and safer. No rotator cuff and wrist injuries.
FastFreddy, I'm 255lbs, 6ft 1". I can bench 320lbs 4times right now. I warm up with 225lbs (4 45 plates + bar) for 20 reps for chest. I've already maxed my gyms pulldown machine for back @ 300lbs. No mechanical advantage there, casue setting the machine @ 250lbs, I can fully suspend my body weight by holding the bar and the weight does not budge. So I'm mixing in both max reps (3 sets) and negatives (3 sets ~6 reps). I'm at 260lbs for back using negatives. I think I've plateaued on shoulder dumbell side raises. I'm stuck at 47lbs 6 reps (alternating between L and R. using Weider adjustable dumbell plates) per dumbell for about 5 weeks now. I really don't see a benefit of going to 50 lbs mostly becasue I'm afraid of injury. High reps low weights.....that just screams of wear n tear and injury for an already poorly blood supplied joint. Don;t know yet what I'm going to do next for shoulders.
For legs, I alternated between single leg (current 185lbs) 6-8 reps, and both together (current 225lbs) 12-15 reps. I think legs benefit from high reps, and it amazes me how one can quickly become breathless doing leg exercises. Gets you sweaty and the blood pumping really fast in the first set.
I like the system I'm using. I'm lifting progressively heavier, getting stronger with minimal injury and joint issues, as well as seeing changes in my body. I cycle (heavy weights vs negatives) every 3 weeks and I havent skipped a workout (or thought about skipping) since the new year.