I drove the southern route two years ago and wouldn´t advice against it at all ! Went over the border 50km north of Porto Novo at "Pobé" where everything was absolutely relaxed with no hussle involved. Of course everything is kind of chaotic with people screaming around at eachother and the guy on his "200l-petrolbarrel-vespa" gets a beating for trying to speed over the border without stopping but relax, its Nigeria ! ;-)
I although have to agree to "roamingyak" that the Parakou area in Benin is fantastic too !!!
My route was ---> Ilaro - Ifako Ijaye - Lagos - Ibeju Ode - Benin City - Warri - Port Harcourt - Calabar - Ekang ->

Cameroon
There are plenty of police checkpoints and some of them definately demand for a gift ( same thing on the northern route through Abuja as i´ve heared from friends ) but generally no problems if you ain´t got one for them and make your point clear. After a while you develop a way of talking and dealing with officials and for me joking in many situations worked well to make friend among the at first even dodgiest policemen ( "that bird last night took all my money, but doesn´t matter cos Nigerian women are best in all africa" , " ah ah, these guys at the last checkpoint were so hungry that there´s no gifts left for you, next time my brother" ) !
I stayed at local peoples places a lot which invited me, camped in compounds and really enjoyed the friendly, energetic, hospitable and well educated nigerians a lot !
The south is amazingly beautiful and although I went to all the "hotspots" where there was trouble and kidnappings some years ago I had no serious problems where I got in danger. Depending the kidnappings I talked to many people and the issue with this is pretty much solved due to military presence and agreements with the government !
Of course I am not pretending its absolutely safe, cos it isn´t ! Stay alert, don´t trust everybody and have a safe place before it gets dark ... but not on an higher level than in other african countrys I would say. I´ve arrived in Benin City and Port Harcourt in the middle of the night due to breakdowns/bad timing and although nothing has happened to me + i managed to find a place to pitch my tent for free its not something i would advice !
If you take the northern road you also would miss out on Lagos which is quit an experience compared to the sterile and boring Abuja. Its the biggest and most charming urban shithole i´ve been to so far and i loved it ! Don´t get why every single time Nigeria comes up here the "Sheraton" is mentioned as if it would be a paradise in hell you have to reach the same day you cross the border to be "safe". Free and safe camping is possible in thousands of places and talking for me the adventure of finding them is one of the reasons i travel ! No need to get any visas there as for Angola you won´t be lucky anyway and Cameroon is possible in one day in Calabar too ( can highly recommend the monkey sanctuary and carnival they have there once a year )
Always when it comes to Nigeria the only thing you read about on the HUBB is bad stories, which I think is a shame ! Dead people lying on the street, kamikaze truck-drivers, bandits at roadblocks, corrupt police everywhere, cops "wearing sunglases to protect themselves agains your pepperspray when they try to rob you" ??!! The traffic is crazy, but not any worse than Ghana for example.
My advice --> don´t judge the country before you went yourself !!! Its easy to get a bad opinion and scared as nearly every local in west-africa warns you of Nigeria and its "bad" inhabitant ! Mostly they are people that have heared rumours and bad stories from other folks and have never been there by themselfes.
Of course there are travellers that had many negative experiences and they are very welcome to post it on the HUBB, but I didn´t find too many recommendations so this is my contribution !
Some pictures of the "southern route" :

( entering Lagos in the morning dust )
Cheers,
Chris