I'm 5'10", and weigh over 280 lbs. My crew includes a guy who's 5'8" and probably weights 130lbs. Both of us wear this harness and are stoked to have the load of the tool taken off our back, and redirected to our waists and shoulders.
It has a ton of buckles and straps which can be confusing at first, and is a bit clunky to put on.
It can work with any weed eater as the attachment point is a J-hook which can take any ring. Thinner rings/line can come out during use, however, this is fairly infrequent, and placing a ring into the loop is a simple matter which doesn't require you to work or manipulate any mechanism. Removing the ring is also straight-forward as there's a bit thumb tab at the top of the J-hook which you lift as you lift your ring, and it creates a nice clean opening.

I bought this for my husband (6'3" 240lbs, waist 38-40) to use, as he has had multiple back surgeries and now lives with an implanted spinal stimulator. He absolutely loves it and says he could weed wack all day if ourb3 yr old would leave him alone that long! Well, on a whim one day I said I would help him with some cutting while he tackled some other outdoor chores, I am a petite 5'1" 125lbs and don't lift weights or workout. He convinced me to use it and actually strapped me for my first time! It does wonders! For someone who doesn't usually have back issues, I was just as sold as he was!! This is well worth the $$. I was able to hold our heavy Stihl without problems and for a couple hours longer than I would've tried on my own!! It is very easy to adjust and fits both of us comfortably with room for further adjustments. I am now a confident pro at securing down in this harness, and I especially like the dual release latch!
I have since recommended it to several neighbors that adjoin our property (we're each on approx an acre of not-flat land). If you're debating which type of harness to get, just buy it! It will save you tons of back aches!!

As far as fit goes: I am a big guy, fits me well (6' 250lb). The belt strap might be a little short. With a jacket on I have about 3" left. The directions are pictorial. I like them. There are three holes in the back brace for med, Large, and XL. you can see them in the image just above and below the strap assembly, it is set on Large (the upper strap assembly can be moved to one of these three holes). In other words: The strap assembly moves up and down the back brace using these holes to adjust.

As far as function goes this thing makes it possible for me to use my Stihl FS110 trimmer for long periods of time. I like it very much.

I never had used a weed whacker or harness until when I bought this harness but I had been watching gardeners use weed whackers without a harness. They seem to hunch over a lot which would seem hard on the back. Also I used my new weed whacker without the harness a couple of times, first, and I could tell there was overuse of the lower back and the shoulder, elbow, and wrist joints. I also didn't like the vibration from the machine going through my joints. I just couldn't see this as healthy in the long run. With this harness I was able to stand in a healthier posture, transfer the weight of the weed whacker to the harness, and overall body. Using this harness resulted in less gripping, so less vibration into the joints.

Also, I wanted the whacker to be clipped to a harness so it couldn't buck back towards me violently. A YouTube video demonstrated how dangerous this could be. Especially if I decided later to use a vicious looking grass cutting wheel.

Note to the wise. Be sure your weed whacker has a METAL clip bracket on the whacker shaft to clip to this harness and a METAL clip into the harness. I first used a nylon cord looped through itself on the shaft and clipped the other looped end to my harness. The cord did occasionally come off the harness clip whenever I lifted the weed whacker higher than the harness clip. When I used the PROPER shaft metal clip and metal carabiner, it never came off. If you are planning on using that vicious saw blade wheel, I would NEVER consider taking a short cut by hooking in with nylon cord. If that sucker bucks back"¦.I don't want to think about it.

This has a great emergency quick release. Takes some learning how to put it on the first time and you might want someone to help although I didn't. Definitely should read the directions.

Lots of adjustment room for longer and shorter people. I'm a short backed 5'2" person and was able to make the proper adjustments. A pain to adjust the first time but once understood is easy and all straps, after that, can be clipped together while maintaining adjustments. Easy to remove with the clips.

Sturdily made. Have used several times. I chose the Husqvarna harness because I wanted it compatible with my Husqvarna "trimmer" but I don't see why it couldn't be used with other whackers since one could get an adjustable sliding eye clamp and carabiner and put anywhere on the shaft. It also has a nice flat, protective hip pad which prevented me from getting banged and poked, and kept the shaft in a good position. I noticed I started swinging the machine with my whole body rather than strong arming it.

As many other reviewers have noted a standard shoulder sling just doesn't cut it. Well, maybe it does cut and bruise your shoulder! I use my 4-cycle trimmer with various attachments to keep the growth on my steep land at bay. After an hour or more with the should sling my shoulder would be sore. The Husqvarna seemed a much better way to carry my trimmer and the reviews were more positive than for other harnesses. I've now used the harness three times and it is a large improvement over the sling. As others also noted there are multiple adjustment straps but I found it easy to figure out a good fit. I'm 6' 2" at 175 lbs, pushing 60 years, and found I like the "small" setting best. After about 5 hours of use over three times use I can say the harness makes my trimmer seem much lighter and more easy to maneuver. The trimmer clips on/off the hook easily. I'm out in the sticks so subject to minimal neighbor stares as they figure I take weed-eating seriously! I'm glad I bought the Husqvarna harness.