For me, a panel upgrade is an easier pill to swallow than a plane with other, more permanent shortcomings. For example, a 1500 lb useful load is never going to turn into 2000 lbs. And sometimes the shiny new panel can be scary ... why did they put all that money into upgrading the panel just to sell the plane? Were the last 5 annuals all over $100k because every rivet in the plane is coming loose or something?
The panel upgrade on the 310 let me have exactly the panel I asked for. That includes the Garmin EIS integrated into the PFD, which for me is a huge safety improvement because it puts the things most likely to kill me (decaying airspeed, unusual attitude, terrain, and engine problems) all on one screen that even a tunnel-vision scan can fully cover. The cost was 10-15% more than quoted, mostly due to airframe issues that came up along the way and would have been good to fix anyhow. That's pretty good for airplane stuff.
But, that being said, I wouldn't rush out to buy a twin without an LPV-capable GPS and ADS-B transponder. I don't mind being the one who invests in a new panel, but I don't necessarily think it should be the first thing I do with my shiny 'new' airplane.