She's a solid champ choice. People call her broken because she's mobile and deals good damage, but she has low defenses and has to get in the middle of the fight to deal damage, excluding her ultimate.
More important is whether or not Riven is your playstyle, not whether the community calls her broken. If you've tried Riven out before and she's just that awesome to you, buy her right away. If you haven't gotten to try her, but you've seen her playstyle and know it suits you, buy her if you can't wait for a free week. Riven's a solid, fun champ and worth your time.