Awesomeness
Epic end of campaign uber-battles are amazingly fun and what a great opportunity for your players to enjoy their final battles!
I just completed a multi-year campaign playing through GDQ1-7 using a 4e baseline, and enjoyed it tremendously. For the final battle, the epic fight with Llolth went through multiple versions (see the MM listing) and multiple "upon reaching bloodied" and "upon reaching 0 hp" effects. The party was 26th level, and I used Llolth from the books, but just dropped her defenses to 30th level effect - the extra bump from a level gap made it even more threatening.
For the minions, I had two waves.
In the first wave, it was a total setup/ambush which unleashed two dragons, giants, and elite drow on the party simultaneously upon Llolth's disappearance.
In the second wave, it was yochlol's and web golems. The yochlols formed a ring around the outside (ranged) and all used a "shield other" effect which triggered their abilities to protect Llolth. The web golems were deadly brutes that hit like a Mac Truck, and were up tight causing havoc with anyone in melee.
Also, tactically I was BRUTAL to the healers.
It was outstanding. No doubt, 4e created a core-playset, that with limited prep, offered a high-level, and beautifully balanced (challenging) encounter as good or better than any epic tabletop RPG experience I've had.