Decide if it fits your needs...
I use an RB Pro-SD pretty much every weekend in the field. And yes, it is not for everyone, so if you are considering one yourself, you might want to at least hold one before buying. I can give you some of the knowledge I've gained about this system, because I have also heard some negative things about this camera:
Some say the lenses are not sharp - I cannot speak for every lens, because i have heard there are inconsitencies with earlier lens quality, but the lenses I own are nothing but tack sharp (50mm Sekor C, K/L 90mm L, K/L 180mm L-A). The RB can be very difficult to focus for maximum depth of field because of the bellows focusing and the different scale for each focal length lens on the body itself. You need to be careful that you read the scale correctly when setting your focus point and the floating lens element, if equipped. This takes some practice and "feel" and i think this is one of the reasons people say the lenses are soft. There are a lot of focal length choices; I would suggest at least a Sekor C or newer because of the coatings and optical quality.
The camera is big and heavy - this is true. It is also awkward to hand hold, but i use a tripod. This may or may not fit your needs. There are much lighter medium (and large) format cameras out there - different needs for different people.
Like you said, there are also different sized backs available, as well as a Polaroid back, and many other accessories. The RB is built extremely well mechanically and should offer years of trouble free service. It was designed for ~50k shutter releases without service, studio use all day every day, so it should hold up well in the field. It is also all mechanical so you won't need extra batteries, but will need to carry a separate light meter.
I have used my RB in hot and humid weather, and early mornings below 16 degrees F, with myself and tripod icing up on a waterfall, and it has yet to jam once. It may take some time to master this camera, but I assure you if used correctly it will not be a limiting factor in the quality of work you can produce.
Good Luck!