I've had a Pro TL for about 6 years. It was my first step up from 35mm. It's a good, but not great, camera. I've done some wonderful work with it, and will still use it for certain projects. Even though I have the grip and the metered prism, I generally use it with the (albeit dim) waistlevel finder and the winder. I use this setup on a tripod and can carry it around quite freely. It's a light enough kit. I don't think the meter in my prism works very well anymore, and even when it did work, I don't remember liking it much. And the motorized grip makes such a racket, I found it unbearable.
In recent years I've had my doubts about the lens quality. I regularly make 16x20 enlargements from Fuji Acros negs, and they look wonderful. Really, I couldn't be much happier with them. I remember though, shooting some band photos a while back. I was using an xpan, and I had my mamiya with me. I was shooting provia and made a few 'duplicate' shots. Apples and oranges, you could say, but one thing jumped out at me right away from the light box. Those xpan negs were significantly more contrasty, and had a sharpness that the mamiya lacked. The lighting conditions were identical, and both exposures were spot on. That was with the naked eye. Under a loupe it was all the more obvious. Certainly the xpan is a marvellous (expensive!) camera, but it was a bit disapointing to see the difference.
I notice that lack of contrast when I'm developing B&W film as well. If the consensus is 10min with a particular film and developer, I generally tack on +25% to get a normal printing neg. I assume this has at least something to do with the lenses.
Now I've more or less replaced my Pro TL with a Rollei 3.5 TLR. The difference in contrast, again, is staggering. The Rollei negs are quite amazing - I'm hooked on them. But I guess it's like comparing a Toyota Camry to a Maybach. Both do the job quite well, but the latter does it with style.
Another gripe with the 645 is the aspect ratio. It's a bit too squat. Not as bad as 4x5, but certainly not as pleasing to me as 35mm.
I also don't care too much for the film backs. I'm not convinced they hold the film flat enough. Certainly, it's an inferior method to something like the Rollei or a Blad.
I'm not bashing. The camera has served me really well, and quite honestly, I've beaten the hell out of it. I'm not very gentle with cameras - and it's never failed me once, mechanically. I see the price these cameras go for now, and it's a steal - I saw a Pro kit for 350$ on craigslist not long ago. But the Hasselblad kits are cheap now too. I think if I was getting a medium format SLR kit, I'd get the Blad. May as well get the best. Or a 67 of some sort.
No matter, you'll notice a difference in quality between 120 and 35mm film. It's unmistakable.
Good luck!