For those that enjoy customizing/painting miniatures, these provide a suitable paint foundation to expand upon. Rather than having to start from the ground up as you would w/typical miniatures you buy for $5 a pop, you can quickly improve upon the look of the miniature through your own drybrush and inking techniques straight out of the box. Add ground cover and you've got a professional looking miniature that closely rivals lead-based.
Another advantage of using plastic miniatures is their ability to withstand being tossed around more during battle, or in storage. Paint on lead-based miniatures tend to chip more often due to their shear weight and inflexibility when colliding with other miniatures or objects.
The drawback w/plastic is that many long swords on these miniatures look more like melted taffy than strong and dangerous instruments of destruction.
What should be stated in addition to that previous post is that the price per miniature is cheaper (even though it provides fewer miniatures than the entry pack) - the expansion packs DO NOT provide the terrain cards, fold-out map, 20-sided die, or rulebook. Expansions packs are a great way to bulk-up on your collection. However, as you start to collect more expansion packs - you will increasingly be provided with the common miniatures that you no longer want and will still be missing some of the more rare miniatures that you desire. This is the result of randomized packaging. Your only hope is to sell your undesired miniatures individually or hope to trade groups of them for a single rare miniature (others like having tons of common miniatures for mass battles).
I only gave the expansion pack 3 stars because I would like to see the price per miniature to be lower, or for them to increase the number of miniatures per package. It should also be noted that the Harbinger series is going to be the BASE miniature line for D&D for maybe a year. After the next series, Dragoneye, is released - the harbinger expansion packs are likely to become unavailable (although the entry packs should be available for the year).