Really this is a show that will probably give you a bit of a surprise. The premise is quite simple: Best Friends Cyd and Shelby, following an accident in their friend Barry's lab, allows them to travel through time by touching the other and thinking of a time period. Cue wacky hijinks .
So far it sounds pretty familiar and sure enough, upon first sight Best Friends Whenever appears to be nothing more than another of your standard Disney high concept teen shows, light hearted and harmless fun sure, but nothing to special or memorable.
But really, keep watching and you'll be surprised. This show is a lot deeper and more complex than it first seems, the level of thought the writers have actually put into the concept of time travel is more than you would expect. Good examples of this are the episodes "The Butterscotch Effect" and "Shake Your Booty" Both of which take a good look at two questions that probably will emerge in you're minds after the first few episodes.
Likewise this show also maintains a concise story arc, a subtle but strong continuity and a pretty intriguing driving question.
The show also benefits from a cast who effortlessly play off each other. The chemistry between the main characters is organic and feels realistic, the bonds between them feel realistic. Likewise the four main characters are well balanced, likable and entertaining.
Now that's not to say this is perfect, like to many other's of its type it falls into the trap of a superfluous annoying younger sibling (or siblings in this case). Likewise not all the humour hits exactly the right mark, and its pretty clear that everyone is still getting used to this so for the first few episodes. Still none of these flaws feel like anything worse than what can be ironed out with a bit of experience.
All in all, I would recommend giving it a try. It really is cut above your average Disney teen show.