Well....the most compelling reason to highly consider whether OEM is right for you is the loss of accessories and the loss of the manufacturer's warranty. Buying OEM, the manufacturer is NOT supposed to honor a warranty. So, it will be up to the e-tailer to whether or not they want to extend you a warranty beyond the return's period.
I find it difficult to buy OEM for expensive items like a processor or motherboard. For smaller items, hard disks (under $125), optical drives, add on cards, I find its okay because all the shipping charges you'd incurr exercising an RMA would equal or come close to a new item.
Don't confuse open box with OEM. Open box is the same as retail. The only difference is its been on store display. If the item has been used, theoretically, it should be returned to the manufacturer so it can be inspected, repaired, and tested so I can now be classified as a refurbished unit.
Be careful. You really have to read the fine print. Newegg is a great company, but their enthusiast crowd is growing really fast. So they are experiencing a huge amount of returns. Newegg is not sending everything that is returned back to the manufacturer. Some of the items will be resold as Open Box. Some items have 15 days to return, many have NO RETURNS.