Cynthia's Guidance is a useful Supporter from Supreme Victors that is also one of the few versatile ones out there. Its effect is simple: Look at the top 7 cards of your deck, then choose 1 of those cards and put it in your hand.
Unlike most Search Supporters (like Bebe's, Roseanne's, and Cyrus), it allows you to get any card you want (not being restricted to Pokemon or specific Trainers). The disadvantage is, searching the entirety of your deck in using Bebe's, for example, will sometimes give you are higher chance to successfully get what you want, as compared to only searching from your top 7 (although being able to give you a Trainer is a big plus).
Compared to most Draw Supporters (Felicity's, POV, Volkner's), which can net you 2 cards or more, Guidance has the disadvantage of giving you only 1 card, max. The good thing is, you can get any card, and chances are, it's a card that you will likely use, as compared to cards that you get from random drawing (why would you pick a card that you will not use, anyway? Chances are, there will always be a card that you will use from the top 7).
Compared to Cynthia's Feelings, obviously the Feelings will give you more cards (whatever your Prize situation is), but still faces the same problem as the Draw Supporters- giving you cards that you might not need at a particular moment.
Obviously, Cynthia's Guidance's usefulness is increased as you go from mid to late-game, where the deck is thin enough that you have increased chance that the card that you need is on your top 7 cards. This makes it even more useful in speed decks.
Cynthia's Guidance is not really the sort of Supporter that will be a staple in decks since it trades some reliability for versatility. Being a Supporter also makes you hesitate to use it, as well. For the most part, Cynthia's Feelings is a better choice since it has more over-all usefulness. However, Cynthia's Guidance is definitely a Supporter that can get you out of a tight spot when you need it. I guess the only real situation when you would not use this is when you have several Uxie in your deck, which can help you draw the top 7 cards anyway.
Modified- 7/10 (not really as bad as say, Rival or Mom's Kindness. It can be a lifesaver when the opportunity arises)
Limited- 9/10 (any search is great here. The decks being composed of only 40 cards increases the chance of finding what you need)