I picked up a fully loaded BMW i3 REX last week. I owned a Chevy Volt for about 18 months ending about a year ago. I subsequently owned a Ford Fusion Energi.
I will try to point out some comparisons to the Volt as best I can.
Looks: Obviously subjective, but the i3 is more "distinctive" looking than the Volt (versus generic ICE cars). I think it is uglier than the Volt, but the boxy shape offers some advantages. I kinda like the suicide doors too. Functional and unique.
Interior: Feels very roomy and spacious inside. Much less dash clutter than Volt. Fit and finish is much better (at least on the top trim model-- not sure about base versus base). LCD screens seem bigger and brighter. iDrive system (despite what detractors say) actually feels refined and elegant compared to Volt UI-- but does take some getting used to. Seating position feels awkwardly high up, but does provide excellent visibility.
Gadgetronics: Stereo sounds great, but up-optioned Volt system sounds great too. I like the more techie information displays in the Volt in terms of inner workings of EV versus ICE. BMW display is very high-level information only. I really like the i3's adaptive cruise control with start and stop. Very nice in heavy traffic. The i3 will parallel park itself. Pretty cool "wow factor" demo for friends, but that's about it. Outside the car, but worth mentioning: there is no BMW provided web interface to the car. I really miss the Volt's website. So much great info there. BMW's integration with iPhone apps is way better. The eyes-free Siri integration is really handy (perhaps newer Volts have this too).
Overall Comfort: Very compliant ride. Very little road/wind noise. Feels like it is sprung softer than Volt, but higher seating position might be tricking my brain. Can feel a bit twitchy on the highway, which I think is due to skinny front tires and high profile.
Handling: You really feel the lighter weight in fast corners. Big advantage to i3. Much more neutral than Volt. Fun to push the car and get a little four-wheel sliding and then a bit more throttle tucks in the nose to scoot you around the turn. Too much fun! The Volt would understeer under similar abuse. Again perhaps due to higher seating position, but the i3 feels like it takes forever to shift the weight from side to side in the turns. I have not carefully explored limits here, but it doesn't inspire confidence. Volt may be the better S-turns car, but I suspect the lower CG point would make i3 handle better than first appearances suggest. Definitely more fun on backroads.
Acceleration: Squirts away from red lights much like the Volt in L/Sport @ WOT, but holds the thrust for a bit longer. The Volt seems to lose steam around 35MPH, but the i3 pulls hard through 50MPH-ish and gets to freeway speed much faster. The i3 has no Sport or L mode, so you can't avoid a heavily dampened/smoothed throttle mapping, making the car feel less responsive than the Volt. But, with an assertive right foot, it will show its superior power/weight ratio. Nonetheless, passing power in the i3 is nothing to write home about.
Range: I have a lead foot and still get about 70 miles of EV range. I am sure >80 miles would be the norm for most drivers. Transition to REX is smooth and full power is available as long as you keep you average speed in the 70-75 MPH range on flat ground. Sustained cruising about 75MPH and/or sustain hill climbs will quickly deplete the SOC reserve and cut power A BUNCH. In an intentional stress test, I drove the car on REX over Highway 17 to Santa Cruz and saw speeds of 25MPH (in 50MPH zone) before pulling over. This car NEEDS a Mountain Mode (long story here). That said, with some planning and self control, I think most moderate grades can be handled even in REX mode. On EV, the car will happily go 85+ up any grade I have encountered.
Charging: charges at 7.4kW on Level 2 (about 20-25miles of range per hour) or via 50kW SAE Combo DC fast charge at about 60 miles of range in 20 minutes (I've tried it, it works). Unfortunately, they are sorting out a thermal failure issue and temporarily patched the SW to reduce max charge rate on Level 2 to about 5.5kW which is a bummer, but still get full charge in 4.5 hours. On L2 it takes about 21kWh to fully charge (18.8kWh usable charge window I think).
Preliminary conclusion: A better car than the Volt for people who rarely drive more than 70-80 miles a day, as you can do that all EV and have more luxury/fun. When the DC fast charge network is built out a bit more (free until 1/1/2016), maybe I would expand that to 100-120 miles a day max. If you need more range freedom, the Volt is better. But this would be if the cars were priced the same. They are not. For now, you pay full MSRP ($55K for mine-- a loaded one) with only the Fed/State rebates ($7500 + $2500 in CA) knocked off the price. I think the Volt is a MUCH better value for almost everyone, so net-net will be the better choice for anyone who is price sensitive. The superior performance is noticeable, but not night and day. I paid up to be an early adopter and to try something new, so I am not complaining.
I'd be happy to answer any questions anyone has.