Depends on which section your fishing. The park to Baptist holds a good mix of rainbow and brown trout. Baptist to Cedargrove, your main quarry is brown trout with the occassional rainbow & a rare smallmouth. Its not uncommon to catch browns in the 16-18" range, and there are some alot bigger than that. Dont go to light on your line or tippet.
Cedargrove to Akers, your main quarry is put and take rainbow in the 10-12" range and a few smallmouth between Cedar and Welch Spring. Akers to Round Spring, is usually slow fishing for smallmouth and the occassional trout. The smallmouth fishing starts to pick up a bit below Pulltite, but its much better below Round Spring. Have fun.