You should buy at least an issue or two to give them a try. Magog's not too interesting to me, but I've enjoyed some of the Power Girl issues. The latter series is uneven, but overall, I'd give it a B.
The new All-Stars issue that just came out today was above average. The pacing could have been better, but it was an entertaining read. Stargirl and Power Girl have a nice quiet moment together between the action sequences.
I did pick up the new All Stars title but I am still avoiding Power Girl and Magog. I can tell these titles are not worthwhile for me from what I have read on the boards. I have never been a huge fan of Power Girl and I dislike Magog so it's not hard to do. I would much rather see interesting characters like Sand and JJ Thunder get page time than these characters.