They're an accessory exercise and I think if you're starting out or unsure on your form you should probably be dropping the weight.
From what I've heard you should generally be able to do 10-12 GM of about 30-40% of your deadlift max. For me this is about right.
The best advice I've heard on the GM is tight back, unrack the weight, feet should width, slight bend @ knee then slowly hinge at your hips until your as close to parallel as you can get and then explode up using hamstrings and glutes. Doing this I get way way closer to parallel than what is shown in either of these videos. I think if you're knew to GMs then this is how I'd do it.
Personally I still have sort hamstrings 2 days later doing this. My DL is about 550lbs 1RM. I do 5x12 of these GMs @ 150lbs and have some in the tank.
Drop the weight until you can keep your arch and get close to parallel for 10-12 reps.
Especially when you're knew to something, repetition is the best way to perfect it and get strength. Once you've got a groove then I think you can start going for 1-5RM type stuff.
T-Nation has some videos where they Thibs goes through his version of GMs. I think they're closer to what I would want to be doing with GMs.