I have battled with my curly/wavy hair ever since it decided to stop being easy and straight at about age 12. This usually resulted in me having big, frizzy hair, or simply putting it all back into a ponytail. Eventually (about 2.5 yrs ago), I got up the courage to look into and buy myself a straightening iron. My sister already owned a Hot Tools 1190 flat iron (~$30 at beauty stores, Ulta, etc) and liked it, so I decided to try it myself.
I remember the very first time I used it, and how beautiful and straight my hair looked! My new flat iron and I became best friends instantly. I used it for a good 6 months or so before I decided to upgrade to a newer, fancier, more expensive flat iron. The one I bought was twice as much (with suppodely ceramic coated plates for less hair damage) and I swear it didn't seem to give me any better results. From my own personal experience, as well as from the hours of research I did reading review boards on many many different flat irons, the Hot Tools 1190 model is by far the best one out there for its price range.
Eventually I upgraded again (after leaving my flat iron on for 24 hrs straight and frying the plates) to an HAI flat iron ($90, beauty stores), which has fully ceramic plates and great heat controls. I highly recommend this flat iron for those out there willing to spend this much. It heats up in about 30 seconds, straightens my hair beautifully, and seems to do much less damage. It is my new best friend.
Of course, please don't even think of using heat styling tools on your hair without some sort of protecting hair product. My current favorite is Redken Align Ultra-Straight Balm ($12, at salons and beauty supply stores). Another good product that I have used in the past is Sleek.look Iron Smoother (~$15, at salons and beauty supply stores). Some less expensive options are available at drugstores but I haven't tried those yet. There are few things I really splurge on, my hair (and its health) is one of them.