My machine seems to sew great no matter what I put in there, but I like fine thread for most sewing. Partially because I'm lazy and the finer the thread, the more that will fit on the bobbin! I like Precensia or Aurifil for piecing.
I don't like C&C because of price and inconvenience - they're convenient to buy but there's so little on each spool, I have to use 2 or more spools per quilt which is an inconvenience. And since there is so little on each spool the price really adds up. Connecting Threads' thread is heavier and lintier than Precensia or Aurifil so I don't usually use it for piecing, but I like it for quilting where I want the stitches to show up. Whenever they have a sale I stock up - I have 1-2 spools of every color they carry and I like being able to almost always find just the right color for quilting right there in my cabinet, and the thread is inexpensive so I don't mind "stashing" a lot of thread. There's a ton on each spool, instead of using 1-2 spools per quilt, I get 2-3 quilts out of each spool.
I didn't listen to my dealer either. She recommended Robison-Anton for my machine, and gave me a promo box with a ton of it with my machine. I don't like it. It's thick, it's linty, and I've found knots on two separate cones. I'm slowly using the free stuff up and will not be buying any more. It doesn't break or anything like that, it's just no fun to sew with.