I've had a GS-1 for a few years and really like it. I don't have any long lenses to answer your question, though. I don't even have any other MF cameras (save a 90s-era Holga) to compare to. I probably qualify as a step below a hobbyist, so take it for what it's worth.
The deal I got on ebay was quite good - speed grip, standard 100mm lens, AE prism, and a 120 back all in great condition for less than a used Hasselblad body (yes, I was looking for a Hasselblad). The GS-1 is way out of my league, and I am quite happy with it. I basically take snapshots - so a point-and-shoot with an average lens is good for me.
I've not gone with longer lenses, but did pick up the 65mm and may get the 50mm.
The camera is light and reliable. There is nothing truly bad for me, but there are a few tiny little annoyances.
- The battery door is a bit of a pain to unlatch if you have human-sized fingers.
- The focus & aperture rings move opposite to what I'm used to.
- Some lenses have different sized threads for filter rings, so you may need duplicate filters in different sizes, or a step-up/down ring.
- My screen does not have grid lines marked for different backs - so I have to work out what's in frame for 4.5x6, 6x6, and 6x7 myself.
- The split-focus on the screen is horizontal. It's probably just me, but I find diagonal to work better.
- It is electronic. Actually, I don't mind this, but electronics are more likely to fail in a manner I can't fix.
- I'm not fond of the LED display in the prism, I'd prefer LCD. Just a personal choice.
- People see it and think I'm a professional, lol. When I take a good photo it's by accident.
None of these should be deal-breakers.
In addition to all the positives mentioned by others (some, like lens quality, far above my level anyway):
- I think it can still fire the shutter with a dead battery - though in a convoluted way and only one speed (I'd have to read-up on this again).
- The "cable release" is electronic. Aside from the general worry about electronics, the remote release is simply completing a circuit. You can plug headphone wires in and fire it by touching the wires together. I was able to get a cheap modern radio-trigger release that works just fine on it.
- It has aperture priority. If I'm doing automatic, this is what I want.