SkyMiles are often called “SkyPesos”–and for good reason. They are generally considered to be the least valuable miles among the major airlines, and the program is not very user-friendly. I hardly use them, but they have actually rescued me several times when they have indeed provided better options and value than the other programs (it pains me to write that).
One-ways permitted? No. Round-trip awards only (open-jaw routing permitted). If you need a one-way award, I recommend looking into another program (American, United, or Southwest).
Partner flight redemption: SkyMiles are redeemable on all SkyTeam Alliance partners, including KLM, Air France, Korean Air, Czech Airlines, (—), (—), etc. Virgin Australia is a non-alliance redemption partner. However, many of Delta’s international partner awards require fuel surcharges, which make them almost useless unless you have a mountain of SkyMiles and a lack of other options.
Off-peak and special deals: If they exist, I don’t know about them. Feel free to educate me in the comments section.
Award chart: Delta miles will generally get you less than with most other award programs. They have spent the last decade angering frequent flyers by constantly inflating their award chart and therefore devaluing their miles to their sad state today.
Routing rules: This is the saving grace of the program that allows you to find a great deal once in a while. One stopover AND one open-jaw are permitted, which can essentially equate to a free one-way flight. For example, I was so proud of my girlfriend when she booked a genius itinerary from San Francisco to Guatemala, Guatemala to Atlanta (stopover for 4 days), and then Atlanta to Bangor. Despite flying all over the place, she was able to get the whole itinerary for only 35,000 miles, the low-level “saver” price of a simple round-trip to Central America.
DeltaPoints’ blog is a good place to start