Is the cold damage feat combo worth it?

[Saeviomagy]

free sneak attacks? how so?

By ignoring the "when you use a power with the frost keyword" line.

Basically, from what I understand, one of the feats gives any target of your frost powers frost vulnerability.

The other one grants you combat advantage over any target with frost vulnerability.

Rogues need combat advantage to sneak attack.

But given how easy it is to get a sneak attack, a 'free' sneak attack is hardly a big deal.