The fact it's Fuel Injected, has it's own body style and the "New kid on the Block".
The 500i is only one of the 4 saws that I purchased brand new and it's a Great saw, but I'm not gonna lie, I regret it.
It's got Great power for the weight just like the 261, but for my application, even with the Stiffer suspension it's a bit wobbly with a 36" bar, noticably lower torque than and consumes fuel at the same rate as a 661.
If you're running it with a 25-28" Bar it's Great, but anything bigger I'd go with a 661 for the Extra Torque and Stoutness.
It has been reliable for me, but eventually will break down and coming from someone who works on his own equipment (unless under warranty), I like things that I can repair in the field and parts that are easy to order.
As of right now, there are some critical parts only Stihl sells through a dealer and which is a big minus.
And where I'm at the 500i costs $1-200 more than a 661 and I figured out the hard way that it wasn't worth the extra cost.
Mad3400