Happy Wednesday to all.
We suppose that it really does boil down to your personal choice as to where to fit this Twin into your deck and your personal style of game play - are you control, combo or aggro ?
First off, just a brief re-cap -
Splinter Twin is an creature enchantment which allows you to put a token that's a copy of this creature onto the battlefield. This new token has haste but you have to exile it at the beginning of the next end step.
An important note however - if you copy a creature with 'X' in it's mana cost, X will be zero with the new token (so forget about Protean Hydra and the like). Also - One will not be able to kick or multi-kick the new token critter (such as Gatekeeper of Malakir).
Here are some cards which we felt was some decent synergy with Splinter Twin -

Wall of Omens for a chump blocker and a card draw ;
Kozilek's Predator to puke Eldrazi Spawn ;
Pelakka Wurm for life gain and card draw ; or perhaps
Broodmate Dragon to squeeze another 4/4 dragon token out ?
~