what advantages do squats have vs. lunges?  leg press with pads on top of shoulders put a lot of pressure on the lower back (from my experience), what about seated leg press?  are squats really "essential" to build legs, would lunges be enough?  how about squats on this? http://us.commercial.lifefitness.com/content.cfm/squatlunge

It's not that squats are essential to build legs, they work so many more muscles than that, especially if you do real barbell squats. We're talking the whole leg, erector spinae, your abs (whole core). Even in some parts your traps and arms get some of the load. It's not the end of the world if you don't do them, but they're an amazing exercise which is why everyone keeps hyping them. It's not just here either, other forums I post on have people raving about BB squats daily.

its not safe it would mess up your knees

http://scoobysworkshop.com/simplemachinesforum/bulking-up-and-gaining-muscle/full-squats-vs-partial-squats/
Covers machine squatting and squatting advantages in general.
Leg pressing is somewhat advantageous if you have a very tall trainee, who'd have difficulties bending during the squats. At your height/constitution, I think squats would be perfect for you.
And no, lunges are not one-legged squats, nor are similar. Lunge is a motorically complex movement because of the stepping phase it consists of, something you don't find in squatting. As I said earlier, do both of them, and dump the machine work. Your legs will thank you.

It's called mechanical advantage, using pivots, levers etc in order to "gain" strength

Mike, if I can toss in my one cent on this one...  Back in Feb 01, I hit a new peak on my BB Squats - 405 for 6 reps.  I was rather proud of that.  I was doing a strong man comp here in Germany.  A week later, I had a really bad accident and it nearly destroyed my left leg.  I wasn't able to work my legs for over a year and they atrophied horribly.  I could only work my upper body and even that was tough.  

Once I got a thumbs up from my Dr, who was a weightlifting German orthopedist, by the way, he and I discussed exercises.  Kind of weird working out with a surgeon, but it was a good experience.  My left upper leg was 2.5 cms narrower than my right.  My hamstring was completely gone.  There was no more nice curve in the muscle.  I could barely do one legged lying curl with 5lbs!.  Pathetic.  My Dr / Trainer suggested lunges and over a long time, my hamstring came back.  Not 100%, but enough.  Nothing else worked my hams like those lunges.

Today, I'm up to 360lbs at 10 reps on my squats and can do 65lbs curls for reps with no problems at all.  I've got pretty good definition in my hams again.  Personally, Brother, I owe it to the lunge.

Sorry that was a lot, but I thought it supported the lunge thing.

Oh, before I forget, while I enjoy anything Hammer Strength makes, that looks more like a shoulder shrug machine than something for lunges!  

~ Good luck!

Me personally,I prefer squats, but I don´t use weights, only highreps( 2x500) then finish up with 10min of delayed squat(hockeyposition?),deep squats are supplemental exercises, and usually do about 4-500(i do them slow and delayed) in total.Ofc I have other exercises for legs too.
To me all that"you need heavy weights and low reps to get big legs"is pure nonsense. then again I have done this for many years.

I disaggree completely, I have NEVER used squats with weight, and have had great results. I do purely classic martial arts training on my legs.