Impact is actually pretty decent. If Fire Pro is a 10, and SVR 2009 is an 8, Impact is a 6. Well worth the $10 at Game Crazy, or really $5 considering you get 1 free game, too.
I have played 6-8 exhibition singles matches, including a couple Ultimate X, as well as the first few matches of story mode. There are obvious flaws with the game. Some collision detection problems arise, but I can count those occurrences on one hand. Twice, while chasing me outside the ring, opponents tripped themselves over the steps. Honestly, that was kind of funny.
Some of the controls are unnecessarily complicated. There's no need for separate buttons for punches and kicks. To differentiate between weak and strong strikes, they could have set weak to tapping the strike button (preferably X) and strong to holding it. LB + Y doing three different things also causes some issues. It wouldn't be so bad if you didn't need it to use your finisher, but the fact that you have to grapple with LB + Y in order to use the finisher is awkward since you have to stand still. Pressing forward or back while entering the button combination causes you to do an Irish whip or other attack.
To summarize, there are some collision detection issues. Some controls are awkward. They're nowhere near WWF Attitude level bad, but also nowhere near as intuitive as SvR's. Everyone's been over the no DQ/countout thing. I don't mind since the game is a fun arcade style title, but I wish there was an option to turn those things on and off.
All that being said, the game is worth the $5/10 at GameCrazy. I'd say it's actually worth $20, maybe $30 if the online play, which I haven't tried, is decent.
Oh, and about the Planet Jarrett achievement: I wonder if anyone has that. I have a friend on my list, and acquaintance of mine who is former indy wrestler who came up with Samoa Joe in the late 90's and remains friends with him to this day. He has Joe on his friend list, yet still doesn't have the achievement. That seems to imply that either Joe doesn't play the game, or that one or both of them dislikes the game enough that they never played each other online. Then again, said friend only has achievements from 10/6/2008 and hasn't played the game since, so I guess it has more to do with my friend than Joe.