I purchased a Stihl FS90 brand new in 2007. I'm not 'in the business', but as a homeowner with a ton of trimming to do I've put literally 100's of hours on it. I've never had a lick of trouble. Every season I give it a new spark plug, air filter, and fuel filter. I feed it a 50:1 mix of high octance gasoline (ethanol-free) and Stihl HP Ultra 2cycle oil.
I wouldn't trade it for anything else out there. I love the power of the 4-mix engine and it performs wonderfully. You would not be disappointed.
That said, any reptable dealer will have a late model 4-mix for you to try out, so I would recommend doing so.