I love ours. We bought it last year and the first time we used it, we pitched in torrential rain and sat in it all evening thanks to the rain - no bedroom at that point, but none of us got wet or anything (until DD1 started throwing up, but that's another story!).
Bought the bedroom for the next trip and pitched in drizzle, and it then proceeded to rain properly all evening and all the next day too - have some lovely pictures of loads of children roasting marshmallows on the huge campfire (started when it was still only drizzling), all in full waterproofs . We adults sat out all evening in our full waterproofs round the campfire too! Anyway, the bedroom was lovely - made a lovely cosy space for us all with a lovely living area at the front.
I made bunting for it this year, and bought some solar powered fairy lights for round the door. Didn't get any rain this time round (first trip of the year last week) - hurrah!
We bought a few £7.99 rugs from ikea for the living area too, which I love .
Seriously, the groundsheet is such a non-issue and I think bell tent only commissioned the SIG and then the ZIG for people who were moving from modern tents to bell tents - it's our first tent though, as adults, so weren't used to the newfangled SIGs on modern tents.
Don't forget, the bits that the guys attach to on the bell tent make a sort of overhang over the bottom of the tent, and any rain that does flow under it, goes under the rain flaps that you tuck underneath the groundsheet.