Here's a Tennis Week profile of him:
There are a couple other short videos of him playing if you search "Ryan Harrison."
Never seen him play, but I've read very positive things about him from those who have. He's said to have a good all-around game. Likes to come to net. Pretty aggressive. Above average athleticism. No glaring weaknesses. In addition to his Australian Open run, Harrison was a quarterfinalist at the Nationals in Kalamazoo and a quarterfinalist at the Orange Bowl. He played Paul Capdeville in qualifying at the Sony Ericsson Open in Miami and lost respectably, 7-5, 6-4. He may be the #2 player in his age group after Bernard Tomic, the 15-year-old Australian wunderkind. There are some others in the discussion, though: Filip Krajinovic, Yuki Bhambri, and Giacomo Miccini. Harrison hasn't played Tomic or Bhambri. He beat Miccini in straight sets recently and lost to Krajinovic 7-6 in the third at the Eddie Herr.
Ryan's 13-year-old brother, Christian, is considered better than Ryan at the same age. I believe their father has even said as much. Guillermo Vilas supposedly watched him at age 10 and said he was the best 10-year-old he had ever seen. Christian just won a fairly big 16-and-under tournament last week.
Tennis Week did a profile of Christian as well:
None of this guarantees anything, but they are certainly ones to watch. Ryan is probably the best American prospect under the age of 18. Rhyne Williams is very talented as well, but he hasn't really done much to speak of lately. Not sure what's going on there.