Jim Wendler is one of the powerlifting greats who has walked the walk and now writes to help other aspiring lifters along the way. Wendler is one of the few people who has squatted 1,000lb, so he might have some experience we can learn from. In this article we'll look at the program itself (briefly) and then my thoughts on it.
His program 5/3/1 is a fairly simple 4-week cycle, that looks something like this (other variations of the program exist, so this isn't the only version of it out there):
Wk 1: 5x65%, 5x75%, 5+ x85%
Wk 2: 3x70%, 3x80%, 3+ x90%
Wk 3: 5x75%, 3x85%, 1+ x95%
Wk 4: Deload
You'd start the program with ~90% 1RM as your training max, so all of the above %'s are based on that 90% figure (ie 5+ x85% = 5 or more reps at 85% of 90% 1RM), and then in each training cycle add a tiny bit of weight onto your training max.
You would train 4 days a week (ideally), with each day dedicated to a different lift (the vanilla lifts of the program are squat, bench press, deadlift, and overhead press), followed by a pile of assistance work. There are various assistance work protocols covered in the original 5/3/1 for raw strength manual, and I assume there are many more assistance work protocols covered in the 5/3/1 for powerlifters and 5/3/1 for bodybuilders manuals. One of the most common assistance work protocols is BBB: Boring But Big. This protocol is to simply take two exercises (one of them being the main lift) and do them for 5x10. Wendler recommends starting out light with the assistance work, so this 5x10 might only be at about 50% training max when you begin the program.
So, with a 1RM of 200kg (training max of 180kg), a squat day in the first week with BBB might look something like this:
Squats: Warm up; 5x117.5kg, 5x135kg, 5+ x152.5kg.
BBB: Squats 5x10x90kg; GHR 5x10.
As I said before, after each cycle, you would increase you training max slightly with each cycle of the program. You'd be looking at about a 2.5kg increase on the upper body lifts and a 5kg increase on the lower body lifts each month, which doesn't sound like much, but adds up to 30kg/60kg over the course of a year (on paper, at least).
5/3/1 is very neat and tidy, with clear progression built in, recovery, and auto-regulation (by seeing how many more reps you can achieve after the first 5, 3 or 1 in the top set). Progression, recovery and auto-regulation are good things. There also tends to be a lot of volume in the assistance work (although that depends on just what you do for assistance), which also has its benefits. There's definitely a lot of praise I can give for to this program.
However, it's been my personal experience that 1 set demonstrates strength, and additional sets build strength. While everything you do in the gym has its contributions, there's a very clear single set in each session which really counts, and that's the heaviest set of the main lift. The only lift of mine that's challenged this notion is my deadlift, but at the same time that 1 heavy set of deads was allowing me to improve on them next week, I was doing roughly 10 heavy sets of squats throughout the week, plus a lot of pull ups. But whenever I've worked with programs that have used 1 heavy set for everything, nothing has improved. 2-3 work sets, on the other hand, has been a formula for success with me.
Running in that same vein, I have done 5/3/1 before, and would you like to guess where I did make improvements? My assistance work! The top end of my strength plateaued throughout the program, but my mid-range strength increased each week. And of course, it's in the mid-rang that I was doing lots of sets.
One of the aspects of the program that's both a big pro and a big con is that you get to decide what to do for assistance work. On the bright side, this results in a very personalised program. On the down side, if you aren't firmly grounded in how to balance out a program in a way that is conducive to your goals and that prevents injuries, you don't have much place in choosing assistance work you'll do.
It should go without saying, but what works for me won't necessarily work for you, and vice versa. So don't take my experience as proof that you shouldn't try the program -- many people have and in doing so have had great results.