The 659, in my opinion, was kind of a star in the the 6xx lineage. Audioholics bench tested it, and it was impressive. I own the 657, and use it in my bedroom system and I love it. I only replaced it, because I wanted the wiring simplicity of HDMI. Wiring is the ONLY simplicity of HDMI! It's a pain in other ways, but it works for the most part.
If you have sufficient inputs on the 659, I would stick with it for awhile. Lossless audio is of questionable value, see the AVR FAQ for a discussion of why.
Reasons to upgrade your 659 -
* You need more inputs
* You have a Blu-ray player, and you are losing sleep over not having lossless audio (I think some people do
* You want better room correction ( Maybe look at Audyssey equipped receivers, but the newer versions of YPAO seem a bit better)
* You want Dolby Dynamic Volume (Which sounds great, but Yamaha has not gotten this yet; Yamaha's adaptive DRC is not the same thing either)
I personally went with the Yamaha RX-V3900. But I wanted it's specific feature set, and was willing to part with over $1500. Most people aren't.
One option is to get a newer model, like the 665 and then buy an amplifier like the Emotiva XPA-3 ( $600 not including shipping.) That way, you can live with the lower powered Yamaha models (when compared to your 659.)