I replaced the lower ball joint on my buddy's 5th gen Accord. I have a ball joint press that I got at Harbor Freight ($30) that worked fine. Here's the problems I ran into: I couldn't adapt the ball joint press in a way that allowed me to press it the old ball joint out. So I had to remove the knuckle off the car, flip it upside down, and use a mini sledge to tap the old ball joint out by banging on the ball joint bolt. Now you could in theory tap the new ball joint back in, but the press is generally considered more foolproof. The design of the knuckle wouldn't allow to me use the press alone to press it back in...I needed some type of spacer to make it work, and two large sockets and a flat prybar to bridge the gap worked like a charm. So basically I had to rig it bit to make it work, but in the end the ball joint press worked like a champ.
Of course the other alternative would be to remove the knuckle, take it to a shop with a large press and have them do it.
Here's my press (with a ball joint releasing tool also pictured sitting on top...which is worth it's weight in gold haha...this thing has released ball joints I couldnt' get pounding the crap out of)
Hope that helps.