I gave up on Challenger 2000 before 2000. But I have it, and a pretty complete set of digests. Played Challenger, and Challenger II for about a decade before finally giving up on the series after a few attempts to play with the Challenger 2000 rules in the first half of the '90s.
As I recall, Bruce Rea Taylor, the author, passed away before completing the re-write that became Challenger 2000. So the rules had gaps that other people tried to fill-in, with some un-even results.
I found that the series had gone too far into yet-another-dice-role-with-yet-another-chart mania. Every new development or consideration became another dice role you had to make. 4, 5, 6 8 times you threw the dice to complete a single process (like a single tank shooting at another single tank). I think the charts and game mechanics needed a complete re-consideration, not just another set of add-ons (all IMHO of course).
Another problem is that the rules are now pretty much un-supported in their printed/published version.
If you like the detail and game-play of the Challenger rules, you might like these alternatives:
BGMR: "Battlegroup Modern Rules" This is a further development of the Challenger rules. It has taken on a life of its own through a UK-based set of gamers who continue to play and evolve the set. There is a Yahoo group for these rules:
http://uk.groups.yahoo.com/group/BGMR/ IIRC the rules are free. They should look pretty familiar to a Challenger player.
Airland War: Mobius (of this forum) published his original Panzer War WW2 rules in about the same timeframe as Challenger. But he has kept with the rules, contiued their development and improvement, and created a modern ruleset from them. His rules are at a similar level of detail to the Challenger rules, but I found them to be notably more playable. Actually, the improved playability of Mobius' approach allows a better level of detail -- details on armor facings, armor types, ammunition types, are all handled in a level of play that is at least as good as anything I experienced with the Challenger set.
His rules can be found here:
http://www.panzer-war.com/
One of the best parts is that Mobius' rules are also FREE. Free is good. Saves you money, so you can buy more tanks!
But a caveat ... I have played the PanzerWar WW2 rules many times, but never actually played a game with the Airland War rules.