First, do you have a use for StarPoints? You can use them for free hotel nights, in the most basic, standard room available and for airline miles. Compare the cost of the points to what they would buy you in hotel stays or airline travel.
For instance, we're planning to stay at the Sheraton Kona next March, and it will cost us 10,000 StarPoints per night in a Mountain View room for which we could pay cash for as little as $196 per night. If we were to stay 9 nights for 90,000 points vs. $1764 cash, buying the points for $1875 would not be a good deal. Conversion to airline miles has a similar cost/benefit ratio. Right now, there's a pretty sweet deal converting points to miles with American, so YMMV, pun only partly intended.
We earn our points via spending on the SPG Amex, for now. When the Marriott takeover gets more real, we're going to reassess that, but as we're in the midst of a huge home reno project, we're racking them up quite nicely.