I ride my Quick CX Ultra on singletrack as much as I can. It has a 80mm headshok, which is made by Magura and was Cannondale's best suspension fork until the Lefty appeared around 2005. This fork was on tons of Cannondale MTBs from the late 90s until now.
The problem with it is narrow tires,
which I'm attempting to address, and severe toe / front tire overlap due to my size 12 feet and the short wheelbase.
If my new 29er tires fit, I am literally going to HTFU and ride around the toe overlap issue. This only happens at extremely low speed when I'm damn near trackstanding and I haven't crashed because of it, but I'm sure that day is coming. On trails like Grecs80 posted I never have overlap problems because I'm not going slow enough to need big steering input.
If the tires don't fit... I'll be saving my pennies and buy (maybe) or build (most likely) a 29er next winter.
A third, minor complaint of the bike is that the 26/36/48 chainrings aren't that great for singletrack. They're ok but I don't use the big ring at all, and it is extremely rare I need the granny ring either. I would probably go 1x10 if I build a bike.