If you're mostly into multiplayer, don't get it! It's pretty much the same game as counter-strike. Condition Zero is mostly for people really into single-player, as it comes with the official bot, has a single-player mod (half-life style) called Deleted Scenes. CZ:DS is great, I think, with some great death sequences and cut-scenes, complete with bosses for some levels. Condition Zero multiplayer has very few enhancements from cstrike 1.6. The CSBot's nav files are used in the new Hostie AI, as well as the place painting for player location, so now your location on the map is also displayed when you chat or use your mic. I'm not sure on this but czero just got a fifth default model for each side, the Midwest Militia for the T's and the Spetsnaz for the CT's, and I don't think cstrike got this 5th default model. Then there is the Condition Zero Tour of Duty single-player campaign, which is fully customizable and the czero community is already releasing some pretty decent add-ons. This TOD mode is Tony Hawk style, with objectives for each map, like kill 4 with ump, or flashbang kill an enemy, defuse the bomb, kill a defuser defusing, prevent hostage rescue 3 rounds in a row, etc... Also, if you have an older computer czero might not run as well. I have a PIII 1Ghz with 256KB RAM and I get about 30-40 fps with 6 on 6 bots, single player. I get about 45-55 without bots. Czero is pushing the limits of the HL engine! For a mere extra $20 on-line it's not a bad deal really...
http://www.area51discountstore.com/cstrike.html
You don't get any of the other mods when you only buy czero either, so you can't play dod, half-life, team fortress classic, etc... I hope this give you a better idea of what the difference is and weather you want these czero enhancements or not.