Well, I have extensive experience with tuning G1. G1 is a good GC, capable of low latency incremental pauses.
The problem is that with a stressed process, at some point G1 still falls back to a full freeze-the-world mark-and-sweep. For 50 GB I've seen the pause last for over 2 minutes !!!
That said, for heaps above 32ish GiB, we still go with our tuned CMS settings and overcommit one or two additional memory modules. It's a lot cheaper than the time it takes trying to tune in G1 on a large heap with a lot of gc pressure.