I bought the Vibrant on launch day and can say that I love the phone.
It is very light weight and I guess that does give it a plasticky feel, but it is light. Compared to my 1st Gen iPhone it's like a feather. For me, with large hands, I always felt like I was going to drop it. The rubber case from T-Mobile works perfect for me though, adding some heft to it and making it feel more secure in my hand.
The GPS issues have been ironed out with the JI6 update. Prior to the update, the GPS would see 10 to 12 satellites, but only lock on to two or three after a extended period of time. Now it see's 10 to 12, and locks onto at least 8 almost immediately. The compass also is much smoother and more accurate.
I personally have never experienced the lag issues that some people complain about. I can't really comment on that but I don't think it is as large of an issue as the GPS issue was.
Then you have the OS and rooting comparison. The G2 is running 2.2 out of the box, but the Vibrant should have 2.2 before the end of the year. I base this on the fact that 2.2 for the international version was posted up, but pulled back because of issues. It shows it is being worked on and hopefully we get it soon. Rooting the Vibrant is simple and painless. The G2 loses root as soon as you reboot the phone.
If you prefer a physical keyboard, the G2 is the one for you. I prefer soft keyboards, so the Vibrant has it there for me.
Overall the Vibrant is a great phone. Call quality is excellent, speaker phone is loud enough to be useful for me, the screen is the best I've seen on any phone, video is smooth as silk, photo quality is great (check out the low light mode), tons of storage space (16GB, and I added a 32GB SDCard), and 3G speeds are great (I average 5.5Mb down, 1Mb up).