I have used the Experian site's credit watch thing in the past. I believe i got to download reports as much as i wanted on there, which i did once a month.
Now I use MyFico which just monitors the score and notifies me any time my score changes. It is $9/month and I get two reports that i can download at any time.
Remember that you also get a free report each year at www.annualcreditreport.com
from each of the big three bureaus.
In the end, unless you think you have something to worry about you can get a 3-in-1 report from any of the bureaus for about $35. So it just depends on if it is worth it to you to pay the extra for the monitoring.