In the old 1.x days, Installer was the most popular among iPhone users until 2.x came out and they lost the competition to Cydia (forever!) due to the delays. The matter got worse when word got around that Installer was just an advertising machine for RipDev but that's another story...
It seems that Saurik is not going to take the chance of loosing to some new app. As per his new Twitter status update, he's almost done with porting Cydia to the new OS 3.0 and is already done with making the QuickPwn Bundle as well, which is not going to be released to the masses soon enough. The only part to be completed is the backward compatibility, which should be done soon as well! Another cool thing is that, in comparison to the Icy update for 3.x, Cydia won't replace/ delete any files like the libgcc thus making it a more secure option.
Status update on 3.x: I finished a Cydia QuickPwn bundle and have been on making the 'Backwards Compatibility' extension fix other packages.
My current understanding of Winterboard on 3.x is that the kernel patches are not all finished in the iPhoneApps.ru jailbreak.