Hi,
as for the road to come into Chad from Niger I can't give too much info, but I guess the other information is already useful enough. As for around Lake Chad, the raining season at this moment is heavier than normal, so indeed forget driving too close driving around Lake Chad - either you will get stuck in the sand, or the roads will still be flooded - or something in between.
As for N'Djamena, living here I can tell you it is not as bad as it sounds in some of the other replies - yet bad enough to be quite careful. Never ever walk around after dark - simple as that... only from the bar/hotel/restaurant to your car and back. If you order a taxi after dark, do it through the hotel or restaurant where you are - maybe more expensive, but worth it. Having said that, there are a few nice places to go to, including Ali Baba which was mentioned in another reply, which is on Avenue Charles de Gaulle - THE street you should get familiar with from the beginning upon arriving here.
As for places to stay, it's true that there is very few reliable budget places - I personally don't know any, but then again, don't have a real need for it either. Every traveller is however welcome to stay at my place, as long as I know it a bit in advance - I am often away on work trips, so need some planning.
In general, Chad is not as bad as it sometimes pictured, but the usual precautions are even more important over here then in other countries. Coupeurs de route is a bit of a national sport, but in the west around N'Djamena it's not too bad. Don't think about going to the east past Abeche - lot going on for the moment, and no safety guarantee whatsoever. Going south is ok, and especially crossing into Cameroun is no real problem - although I hear rumours that in the north of Cameroun they have their own problems as well - but can't give you real useful info on that one...
Hope this helps a bit... and just so you know that for all Horizons readers there is always a cold

waiting here...
cheers/jef