*front note* I did my best to avoid spoilers. Odss are if you were going to play the game, you have already, but be warned now. The following may contain spoilers.
Lets get one thing straight right now, I really enjoyed GTS 4. I played though it twice and got far more than my moneys worth.
I however, I will NOT be buying the DLC. I can't say that I really disliked Niko as a character, in fact, he is probably the one I sympathized and connected to the most despite his multiple personalities. It pisses me off to see a character who ripped niko off and disappeared after being involved in two or missions get his own story.
Rockstar gave so much potential for advancement in the storyline with main villians still alive, etc. and now it seems that they've just decided to let it all rot with unanswered questions.
Annoying unanswered plot elements aside, why Johnny? He was possibly the least likable character still alive (depending on who you killed of course) and only showed up in a couple missions to ultimately rip Niko off. For example: Why not Packie? He was a developed character and had a past and future ahead of him.
Ultimately I see why Rockstar changed to a character with no back story who nobody gives a shit about. With all the choices in plot (mainly the last choice) they would need to alter the content depending on what decisions gamers made. This would take longer and cost more and rockstar wants money now.
Irritated ramblings aside, I don't personally like biking, especially in a game like GTA where crashing into anything results in you flying head-first over your handle-bars and ragdolling for 60 seconds. Playing as Johnny (who I may remind you is a member of the lost biker gang) I expect many missions involving biking and biker gangs which, while in some settings has the potential to be fun (games like roadrash come to mind), I don't think it'll be very fun in GTA.
To summarize quickly, Bad character, bad gameplay, bad reason, bad choice. I'm not planning on spending any money at all.