Here's another yes vote for Weed Eater's Featherlite series-especially if weight is the primary concern for you. It's easy on the wallet too.... (Weed eater also makes Poulon, Poulon Pro, etc.)
I've been using my wife's 12-15? year old curved shaft 18cc Featherlite (about 7-8#) for most of a season now, (previously replaced the fuel lines with the new type, longer lasting hose) and it's a great little machine within it's limits. For it's engine size the cutting power is initially hard to believe, of course, it's spinning only ONE cutting line with the associated lower wind resistance to overcome. On the other hand, all the available power and impact of cutting is directed to that one line so it does tend to break more often under the same cutting conditions as a twin-liner-it's doing twice the work. It cuts a bit slower than a twin line when in tougher stuff than my larger displacement trimmer would with the same line, BUT OTHERWISE much of the time it cuts as fast as I'd be moving with the larger machine. She had the plastic Grass Gator type head with blades on it but I put the original head back on to use with string. It started out with a SINGLE .065 line originally, but I bumped that up incrementally through a couple of sizes by simply drilling out the spool's line hole and very slightly enlarging the aluminum grommet hole and have been running the red triangular type .105 line exclusively for the last few months (most of our trimming isn't grass, but much tougher, often woodier stemmed and other line snapping stuff including Kudzu and honeysuckle-when doing grass the line seldom breaks) the line doesn't seem to be lowering the top speed much due to increased wind resistance (or at least not from a cutting efftiveness standpoint).
It runs about 1.25-1.5 hours per tank for me while cutting our taller stuff mostly along fence lines, embankments and creekbanks, and that means running most all the time at above half throttle-and most often 2 or 3 sessions per day for the few days each month that's necessary to keep up with the closer areas of our acreage nearer to the house and closest barn. The vibration level is fairly low too-but I do wear vibration damping gloves on most days mainly due to the run times.
My main gripe about this little machine is that it's shaft is so short compared to my Homelite straight shaft, so it would probably be harder than necessary on your back for anyone over say 5'6" tall and/ or who needs to do creek banks or slopes, which mean lots of leaning over time to reach further due to it's shorter boom length. In tighter areas it's more user friendly than any straight shaft I've ever used and I always preferred using a straight shaft before this. I have to say that even though it takes a much shorter sweep, the lower weight and increased maueuverability make a big difference when doing long sessions in tight quarters or when doing narrow strips.
A second gripe is that even though the plug stays fairly clean and I regap as necessary, I've had to decarbon the exhaust port twice now, once after probably more 50 hours of my running time, using various 2-stroke oils and a second time as the power began to drop, but this time I didn't wait as long. Because there's still a little oil drippage coming off after shut down, I'm gradually leaning out the mix from it's factory recommended 40:1. Hopefully running a slightly leaner mix will extend the decarboning intervals considerably. One drawback from many newer lawn and garden machines is that their carbs cannot be adjusted for anything other than idle speed. On this one teh carb still has an adjustment for high and low speed leanness, but for our elevation it's as lean as I can get it at high speed without negatively affecting it's effectiveness.
I noticed at a recent trip to WalMart that Weed Eater has 2 curved shaft versions now, this one (about $65), and one of those has a slightly larger engine (I think)-and that one's shaft is about 8" longer than this one (about $80 + tax). They also have 2 straight shaft versions with larger engines and 2 string heads, I think that the bigger/heavier one has a split boom for swapping attachments). I looked at Weed Eater's site and the Poulon and Poulon Pro lines ther have some very similar models plus additional more expensive heavier duty rated ones. If/when this one conks out the longer curved shaft version is probably what I'd choose as an inexpensive replacement for her, even though it's a little bit heavier than this one (she has back problems too). (She recently bought a corded one from B&D to use by the house-and that one is almost identical in weight to this one-plus the boom is shorter. It may be going back.
(My straight shaft 31cc Homelite ate it's rod bearing, and the fix is to replace the entire rod/piston/bearing assembly plus the various gaskets , etc. involved in doing that. Also, the carb had been acting up before then, so I didn't immediately opt for $60-$80 worth of parts to fix that crank problem which may or may not result in a decently running trimmer without reworking/replacing the carb. I'd been looking at Husqvarna and Stihl as replacements, but after working with this little Weed Eater this last year...........I 'm not as inclined as before to move upmarket.)