Githyanki are good, provided your DM lets you use them. They're a Monster Manual race, not really intended to be used by players. Run it by your DM first. I'd personally allow it, but your DM may feel differently.
Since it's a MM race, there's very little 'support' for it, meaning you won't find githyanki-focused feats or paragon paths. But, honestly, Wizard/Mage has so many options open to it that you probably won't miss your racial options.
Githyanki used to be kings of the Wizard/Mage, Swordmage, and Con-based Warlock classes because they were for a very long time the only race with a bonus to both Int and Con. Now, though, both Genasi and Warforged can be Int/Con, so their appeal has lessened. Still a perfectly fine choice, though, if you can get your DM to agree to it.
Telekinetic Leap is pretty solid. I've heard that it's better used on your allies than yourself, though, for some reason.