I've played Star Sapphire a couple of times with HU's main deck cards but can't figure out how to effectively use her. Is she kind of weak? Or really advanced?
If you can get a good number of Equipment she offers awesome deck cycling/manipulation. But it helps if you have a good idea of just what's left in your deck too.
Ease of use I'd say she's on the difficult end, takes a fair number of Equipment and finesse to use properly.
Power-wise she's probably in the middle.
Early game, before you get much Equipment, keep anything good on the top, pitch anything weak to the discard pile (Starters mainly, especially Starters).
Later (if/when you get a large amount of Equipment) better cards go to the bottom, worse cards get discarded. You want to be seeing (what started on) the bottom of your deck ASAP.
Legion Flight Rings are the best/cheapest card for her, get them if at all possible (note: you do card text first before 'trigger' effects, so you'd Draw a card and then use her ability).
You can also use her ability to 'peek' at the top card of you deck. This comes in handy for the HU Locations.
You can also use her to help set up card combo's. Examples (from HU):
Buy Manhunters. Early Manhunters go to the discard, the last couple stay in the deck.
See a Plastic Man/Kyle Rayner/other-Discard-dependent card early? To the bottom!