I own 7 acoustic guitars, play daily and go through a lot of strings. Martin 80/20 bronze consistently deliver the feel and tone I prefer whether I'm performing live, in the studio or playing strictly for fun. Shoreline's great prices and outstanding service make it easy to stay tuned.:)
First off Shoreline is by far the best company to buy strings from. I know from experience buying strings all over the place, including dreaded guitar center. Free shipping and John sends a nice thank you note even when I buy the 3 dollar cheap Martins. And his son is funny and will often write funnier notes to send along with string shipment. Now, I doubt I will buy the $3 Martin strings again, as while they sound great on my old Guild D25, the alloy tarnish on fingers is terrible. So I give 3 stars for the M540 and M140. I plan on giving the Martin Retos a try as well as the Martin SP Plus. Light or medium gauge, either way, try them all. I love the gritty and meaty tone I get from Martin strings. I do not like coated strings, so Elixir is out, but D'Addario and John Pearse, all great strings, but they are tame to me vs. Martin strings, which just have more power for strumming. The word "raw" comes to mind, like garage band rock, they never break and even the light gauge (.12-.54) really moves the top. My Guild is all mahogany so I need a lot to drive it, and Martins just do it so well! In a pinch, sure the M540 and M140 are good, but rather, spend a couple extra dollars and get the SP Plus phosphor bronze strings. Great woody, powerful, gritty, meaty, jangly tone that doesn't break.
Martin always produces great strings that bring out your guitar's natural tone. Mind you, these lower end 80/20's don't last as long as others, but there is no sacrifice in tone - just duration of the use of the strings. Would buy again (and have bought again).