An immersion blender is one of those pieces of equipment that can seem silly and not very useful. However, once you own one you can't imagine cooking without it.
In modernist cooking an immersion blender is needed to blend in many of the ingredients to ensure they are evenly mixed. If you already own a standing blender it can be used instead in many applications but the immersion blender is often more convenient.
In traditional cooking I use my immersion blender to puree sauces and soups directly in the pot, without having to get my blender dirty. I also use it to create purees from fresh vegetables and to quickly make vinaigrettes and mayonnaise.
Almost any immersion blender will work fine and they all are pretty similar in power. However, you may want to consider getting one with a whisk attachment, especially if you don't have a whisk attachment on your standing mixer. In that case the whisk attachment on the immersion blender can be used for most modernist cooking applications where whisking is required.
There are two good candidates for an immersion blender with a whisk attachment. There's the Breville BSB510XL Control Grip Immersion Blender. This is a high quality unit that comes with an ergonomic pistol grip for easy control. It has an 8" immersion depth for deep pots and 15 adjustable speeds. One thing I like is that if has a non-scratch guard on the bottom to prevent chipping and scratching of non-stick cookware.
The Breville hand blender has an adequate 280 Watt motor and comes with a whisk attachment, blending jug and a chopping bowl. This blender is very highly rated on Amazon and was the highest rated immersion blender tested by Consumer Reports. I would highly recommend this product.
The second alternative for an immersion blender with a whisk attachment is the Miallegro 9090 Mitutto 550-Watt Immersion Hand Blender, Professional-style. This hand blender has a hefty 550 Watt motor with 5 speeds and a "turbo" button. It has a great set of attachments so that it can: chop, mix, grind, blend, whisk, whip, and froth.The Miallegro immersion blender is also highly rated on Amazon and was the second highest rated blender by Consumer Reports. The food processor portion of this product appears to be better than the Breville unit.