Put together an RB kit 3 years ago this month. Yup, it's big but too much is made of its size and weight blocking its ability to deliver stunning images. It's a killer portrait camera. Close focuses without pricey macro lenses, extension tubes or diopters. The rotating back allows quick portrait/landscape changes without shifting the camera's significant mass(try that with a Pentax 67 on a less-than-robust tripod/head--scary).
Watch out on CL stuff, especially if you're unfamiliar with the Mamiya RB 67. Go for a Pro S or Pro SD body+backs. The old RB has no double exp. lock-outs--problematic until you get used to the shutter cock/film advance 2-step. They're also getting very old. These were studio gear that usually did long, heavy duty service. A kit with, say, a Pro S body, WLF, 120 Pro S back, and a 90/3.8C lens is a good start. Backs and RB adapters often need new light seals--a fairly easy DIY fix. I'd almost recommend building a kit from pieces rather than jumping at a kit--while the trade-off is better quality often at a slightly higher price, there's also the old problem of "pay less get less" with used gear.
I have 645 and 6x6 cameras but take out the big Mamiya most often. It's not a camera for fast moving subjects. The huge, plasma-like viewfinder with its magnifier allows for very precise focus. Remember that you'll need an accurate incident light meter, too, and flash metering capability if you plan on using strobes. The purely mechanical operation forces you to take your time and 10 shots on a 120 roll discourages carelessness.