I absolutely love fast cavalry, and as such, Saharans never disappoint. Easily spammable, highly useful, very cheap, and a valor +1 province (Cyrenaica, a province absolutely worthless in most other regards): what's not to love?
Recruited from Cyrenaica with a Master Horsebreeder, Saharans can give you a very powerful fast response force. They're the ultimate opportunists: hang around the flanks, charge where an enemy is engaged, withdraw, charge again, chase some routers, return to wreak more havoc. Even only two of these are worth having in your army. Six to eight are terrifying. More are probably too much of a drain on your staying power, though.
That said, in their standard issue form, I like Steppe Cavalry even more. They're slightly more durable with 1 more point of defense and armor, and they have a more powerful charge (6 as opposed to 4). This allows them to bring to bear more of an immediate impact, and can sometimes be useful against semi-heavy units, as well. The price difference is negligible (both units are dirt cheap, basically).
But Steppe Cavalry doesn't have a valor +1 province, so all in all, Cyrenaican Saharans make it to the top.
BTW: If only Alan Mercenary Cavalry were regularly recruitable!