270mg is about perfect for a first time, but dont expect anything spectacular like hallucinations, but you should have a good time. Taking large amounts the first time is asking for trouble, you have no idea what so ever how your going to react to it. It also depends on your weight as well, but dosage makes a big difference.
For example, the first time My girlfriend took it, she took about 275mg, she said it was the best trip she ever had, I mean we had an awesome time, great trip... BUT the 2nd time she did it, she went up to about 375mg... THIS time it was too much, a couple hours into the trip, she was so fuXXed up, she thought she was going to die... I was trippin balls on like a 700mg myself, so I didnt know what to do, but she made it through, after puking most of it up, but I really thought I was going to have to take her to the hospital.
Shes a small petite person though, so Im sure that played a factor.
Basically my best advice is to start out slow, its a fairly safe drug when used cautiously, but I wouldnt take a big gamble just because of that. And if you do go big, make sure you have a trip sitter, u just never know. And on a last note, I read the post above where the guy went out after taking a mega dose, Im sure he is a veteran DXMer, because usually anyone who goes over the 3rd plateau, should in no means go out or be in public, and POSITIVELY do not drive.