This review is coming from a visitor to Berlin with multiple experiences renting, owning and…read morerepairing bikes.
Zweite is a perfectly good place top rent a bike, with fast service and reasonable prices (€10/day). The bikes are step-through cruisers with baskets, front and back lights, a bell, and three gears - all features standard to many bike-share programs. They also come with a lock and a seat adjustment before you get in the saddle.
The bikes themselves, though, are not ergonomic. I've ridden faster, more efficient bikes with the same upright cruiser design. Also, the gears on my bike needed serious work. They took a strong wrenching on to get them to shift down, and even then, the derailleur clearly needed serious alignment. But to be clear, the bike was still perfectly safe and functional, but more or less as a single speed. Store hours are also inconvenient: 1200 -1800 on a weekday. That makes it impossible to get an early start unless you pick up a bike the night before.
Overall, this is a perfectly adequate place to rent a bike, with only a few minor downsides. Use their service, but be sure to plan ahead.