I'm certainly no expert, but I have had some frustrating experiences with them over the years. I had two Weedeater brand ones years ago that were pieces of junk -- I complained to the dealer so much about the first one that he gave me a "good deal" to replace it with a new, larger model. Same complaints -- very, very finicky to start, always walking a tightrope between flooding it and not choking it enough.
I finally replaced the second one with a high-end Craftsman Convertible about 7-8 years ago -- not sure if it was their "top of the line" but it certainly wasn't near the low end. It has started reliably and given me good service, though the string-trimmer head and bump mechanism are about shot. I've already replaced the "bump feed" once and it is due to be replaced again, though I can't complain -- it owes me nothing. I also have a Poulan Pro split-shaft that takes the same attachments -- I thought it might be a decent replacement for the Craftsman, but I'm not nearly as satisfied with it. It is much more difficult to start..
I've never bought a "premium brand" one (i.e. Husky, Stihl, Echo, Marayuma, etc) but am seriously considering it now, if I can find a power head that will accept my collection of "Expand-It" attachments (brush cutter, chainsaw/pruner, tiller/cultivator, etc.) I'm hoping that I can find an affordable split-shaft Husky powerhead that will use them. Because of the volume of woods-line work we've been doing on my retirement property, quite often my son and I are running two trimmers at a time, one with line on it and one with a brush-cutter head, so I like to have two reliable ones.
One thing I have discovered in shopping around is you need to shop carefully: Troybilt, Cub Cadet, Poulan, Craftsman, and McCullough (perhaps more) all seem to be made by Poulan, which is owned by AYP/Husqvarna. (I think Weedeater brand also, but I haven't checked them out based upon by bad experience with them earlier.) However the 4-stroke versions will use different engines -- some have Hondas and some have Briggs. I suspect Poulan also makes the less-expensive ones for Husqvarna, similar to what they do with chainsaws. (The cheaper consumer Husky chainsaws sold at HD/Lowes are made by Poulan, not made in Sweden. Husky MAY have designed them, but Poulan makes them.) Ryobi and John Deere atttachments will interchange with any of the above split-shaft models, as will Homelite and Weedeater brands. Not sure who builds what, but John Deere owned Homelite who made their John Deere trimmers. Not sure if they still do...
Sthil attachments will not interchange, it appears, nor will Echo. I haven't found a local Marayuma dealer yet..
Since I truly don't use them commercially, I'm reluctant to spend $450-500 for a trimmer, but if I could find a good power head in the $250 range or so, that's a different story...