In general, a great way to buy new DeWalt batteries has been to find a good deal on a new tool with two batteries. Often, you can find a deal where the tool, charger, two batteries and case are about the same cost as two new batteries.
However, as you're considering a move to Li-Ion batteries, be aware that while Li-Ion has many benefits such as high capacity and being lighter, they also have some downsides. Other than higher price, the biggest downside is that Li-Ion batteries begin deteriorating the moment they are manufactured. That's why older cell phones, laptops and cordless tools equipped with Li-Ion batteries often don't hold much of a charge after a couple of years of use or non-use. It is just the natural aging of the battery.
The smaller, lighter and more powerful aspects of Li-Ion have been put to good use by most of the power tool manufacturers. DeWalt, unlike most of the other manufacturers, gave 18V tool users the ability to upgrade to Li-Ion without buying new tools although you did have to buy a new charger. However, the newest DeWalt tools have moved to a different style of battery and while Li-Ion is standard, they do not have compatibility with the older style of Li-Ion or NiCad batteries nor the old style chargers.
If you use your cordless tools daily then I believe you can get your moneys worth out of DeWalt Li-Ion batteries. If you're an infrequent or occasional user, NiCad still offers good usability and value. If you're buying the newest compact style tools, Li-Ion is what you're going to get with your tool.
