I've had my Clarity Plug-In for 3 weeks now. I also own a Prius and a Leaf. I'm not sure what you mean by customizing the regenerative braking system. When going downhill, or if you see that you'll need to stop soon, you can use steering wheel paddles to decelerate with regenerative braking. If you use the cruise control and go down a hill, it will also decelerate with regenerative braking. Lightly pressing the brake will, too, though it seems to do less regeneration than with the paddles (I'm not positive about that, though). If you are at a traffic light, you can crawl forward by taking your foot off the brake, unless you have pushed the "brake hold" button. But I know of no customizations per se.
By the way, over on the Volt forum there's a lot of discussion about Clarity. One complaint that I saw there and worried about was that you can't force it to just use the battery. I can understand that concern from anyone like me who owns a Prius hybrid, where it's a real art to keep it in battery-only mode. But with my Clarity, I have to really, really slam the accelerator on the floor it to get it to exit battery-only mode, at least until the battery is used up. In normal driving, I haven't seen it exit battery-only.
Lastly, I've made six trips of more than the 47 mile EV limit, going 65-75 MPH for much of the way; on average, it's given me about 52 all-electric miles
before starting to use gas. So far I'm extremely pleased with this car.
From what I understand, the new Clarity has a customization regenerative braking system. So if you're going downhill, the regen can be maximized of if you're stuck in traffic the system can be put to minimum so you can crawl forward by taking your foot of the brake. Curious how many settings the system has or if it's just two.