I just developed my first roll of Phoenix I reversal after shooting it at EI 16. First developer was HC-110 dilution A for 6m30s, followed by water stop, light fogging, re-wet, and color development/bleach/fix in ECN-2 chemicals.
Mine came out very overexposed and very contrasty. The contrast I expected. The overexposure is likely a metering thing more than anything else. At the end of the roll I shot a calibration target with 18% gray and a bunch of color squares from 5 stop underexposed to 4 stops overexposed in 1 stop increments. The colors came out surprisingly great, it seems. The target that was underexposed by 2 stops looks about right for exposure. There seems to be a bit of a blue cast in the shadows and midtones, but I'll reserve judgement until I see the film dry and on a light table.
B&H still has Phoenix I available for less than half the cost of Ektachrome. I think for any "serious" work I'd definitely shoot the Ektachrome. But if I take one more roll to dial things in, this could be a very fun way to play around with color reversal on the cheap, with surprisingly great (and certainly interesting) results.