My two cents:
Houllier is an ok/decent manager who's had his day at Liverpool and is showing up his weakness and limitations by failing to drag Liverpool out of their two year rut.
O' Neill appears to be an exciting manager who has succeeded everywhere he has gone despite limitations and he's as good as anyone available on the market at the moment. However, I admit that he hasn't managed a top side (although I am beginning to think that his Celtic side are one of the best in the UEFA Cup draw at the moment) nor has he had the experience of managing top stars, with the possible exception of Henrik Larson. He hasn't won a serious trophy nor has he ever had a big transfer kitty to work with and we don't know if he would spend big money well. However, despite all the hype and the people who think he's the Messiah, I highly rate O' Neill. I think he's as good as any manager that we could hope to appoint in replace of Houllier, and better than most.
For every success that Houllier has achieved, there's a failure.
Stu: GH did BRILLIANTLY for us in getting us to the quarter finals of the CL in 2002 (and we almost and probably should have reached the SF, where chances are we would have beaten Man UTD, over whom we had the devil's luck at the time), but we were absolute rubbish the following year when Valencia whiped the floor with us home and away, and we failed to do the business against Basle.
Houllier had modest to reasonable success in France with one league championship and a cup or two to show for something like 12 or 13 years of management. He was an absolute disaster at managing the French national team in the 94 World Cup qualifiers before earning plaudits in France for his technical and administrative work at under age level.
He did extremely well at Liverpool from ealry 2000 until mid 2002, and since then his team has gone into serious decline.
He's not a bad manager, but he is not a great one either, and for every success that people might point to, there are plenty of failures.