660 is a nice saw but gets old lugging it around all day long, had a 460 we loved but kept scoring the jug and piston all the time, traded for a 461 and everyone loves it, don't ask me why, but its the saw everyone wants to use and fights over to get to run, all around handy is what I call it, enough power to get any job done, but light enough to not break your back.
If your wanting a new 660 you'd better get one soon, many dealers can't get them anymore, only the new 661 with the electronics on them, some love them others hate them, never ran one myself so can't say either way. Been thinking about trading for a new 661, but just can't seem to let the old 660 go for some reason, basically it was the first big saw I've had I liked to run and been a great saw for me, call me stupid but don't want to end up with another lemon like many of the past saws have been for me.
I run a 36 inch bar on my 660, 404 chain and its all she can do to run it, skip tooth would help, but I buy chain in bulk and make my own chains, everything but the 36 inch bar is regular chain and I don't want to mess with skip tooth chain for just one bar. We also run some 3/8 .063 chain, and for 3/8's it gets the job done, but won't stand up or last like the 404 will.
We've pulled over 30k board feet of lumber off clearing jobs over the years, another 5k this year alone, hire it all sawed and bandsaw guys are hard to come by that stick around and keep going it seems, I've worn out five over the years it seems, told the one I have doing it now, I'm bad luck, everyone so far has got out of the business who works for me, he just laughed and told me it comes with the type of work and has nothing to do with me personally. The amish near me do pallet lumber only and are not really interested in sawing logs for finishing lumber, quite a few of them have blade mills and not band mills.
Stihl chain is the only chain to get, nothing lasts or stands up like stihl chain does, tried them all and have always ended up back with stihl, but beware, its so hard its about impossible to sharpen by hand, we use a grinder for all the sharpening.