The unit I found last nite at Home Depot is a Lehr, Model 025DC. It uses a common 16.4 oz propane bottle and has a standard "bump feed" string spool which is advertised to be replaceable with various other attachments.
At first, I was disappointed to find it wouldn't run above idle until warmed up for over 10 minutes... and with poor throttle response, then it had no power and would die when presented with grass to cut. I started to put it back in the box and storm over to Home Depot and remind them of their 30-day no-questions-asked-return-policy and give them a piece of my mind...when I saw the little tag in the box which said, "Do Not Return to Store - Call 866-941-XXXX".... so I did ... expecting to talk to Bish-Hab the PunJab. Instead I got "Scott"...in Dallas.
After I told him how disappointed I was with the product he calmly told me that he would help me make an adjustment that he expected to solve the problem. I described to him that if it were a gasoline-model I'd think it ran too lean. He said that he agreed, and then asked me to take a Phillips screwdriver and remove the air cleaner, then unscrew a thumb-wheel and pull out a throttle-body and then turn a brass needle one, complete turn clockwise. Then he had me reassemble it and stayed on the line while I tried it out.
Bottom line: In about 10 or 12 minutes he had troubleshot and fixed the problem and I was out cutting heavy weeds along my fenceline happy as a lark.
The unit normally sells for $219 but Home Depot has them on sale for $99 and I'm recommending you guys try this if you are tired of mixing oil and gas and having to clean a carburetor every season. (Yes, this is a 4-stroke engine and has a 2.5 oz crankcase that uses ordinary 10W30 motor oil. It's quiet and lightweight and seems to have ample power.)
http://www.golehr.com/products.shtml
By the way, Scott doesn't work for Lehr. Scott works for a U.S. small business which handles Customer Service calls for U.S. businesses and uses AMERICAN customer service reps. It's a "transparent" put-thru call service. The company he works for which handled this call on the Lehr product is AWS (American Warranty Service), a small business in Dallas. I think they're on to something!
May there always be and Angel flying with you.
I am proud to say I have only owned one weed trimmer (gasoline) & one electric one ever. I have been lazy and hired my yard mowed & trimmed for many years. I occasionally break out my 1986 ECHO GT-2100 trimmer, but it doesn't have a primer unit on it like most do now days so its a real b_ _ _ _ to get started. Requires pulling the plug & pouring gas in to get the first start. Then it restarts OK for that trim period. My Craftsman selfpropelled 7 hp mower gets used about once or twice a year to cut outside my fence. It doesn't have a throttle (runs full speed all the time), but is easy to start thanks to that primer. I guess the no throttle is a EPA thingie 
And yes, George, Hank Hill would still be proud of you even if you don't drink Bud since you are using propane. I had never heard of a propane powered weed trimer. Hmmm--maybe I could convert my mower to propane???:mrgreen: