My old Pantone branded gretagmacbeth Eye-One Display 2 is struggling, particularly with newer versions of OS X.
Yes, I'd say it's long overdue and time to retire it. Today's needs exceed what that device can do.
After having a look here and elsewhere the X-Rite i1Display Pro seems like a good replacement.
Yes, it's solid solution, and the only one I recommend to demanding pros. I own nearly everything but rely on the i1DP for my daily travels and consulting. The consistency across a variety of devices, speed and excellent shadow detail are all big pluses for me.
1. Can I realistically expect good/consistent results across a range of devices from just one tool.
Yes. Note that i1Profiler wants to know what type of light source the display uses (CCFL, LCD, wide gamut CCFL, etc) before calibrating. It's easy to overlook this, but don't as it's quite important and is what provides excellent consistency across these different types of displays.
2. I’ve read conflicting reports about how good a job colorimeters can do with Apple’s Retina display. What can I expect? What should keep in mind when making profiles.
It's excellent. As always, think about matching the color and brightness of whites onscreen to a white piece of paper under your lighting conditions. This is a basic concept of display calibration that's all too often not understood, IMO.