Calling Hessians "mercenaries" is a bit of misnomer, as it implies that they were simply guns for hire. They were regiments in the service of sovereign governments which were rented out wholesale by said governments to the British. It also implies that they were willing participants, when many were conscripted or press ganged into service.

The individual Hessian soldiers weren't really mercenaries, as they were often conscripted or press-ganged into service. The Princes of these German states had basically turned his nation into a mercenary regiment generator, maintaining huge military forces which they hired out to other nations. Some of these Princes had 5-10% of their population enrolled in military service at any one time during the 17th and 18th century, to the great detriment to the rest of their economy.

The Hessian soldiers were very professional; though, rather unmotivated on an individual level. They were spoken of well in their performance, and generally considered to earn their pay and be worth the money. The initial wave of 16,000 or so soldiers brought over from Europe had been told that the American colonists had rebelled against a fair and legal ruler. These soldiers grew up in a very military paternalistic state where ideas of blind obedience to your sovereign were drilled into their head constantly. They would have had little sympathy for the colonists at first, and when they saw how obvious it was that the colonists were going to lose in the first year or two, they were motivated to end the war quickly.

As the war went on, the companies came to sympathize with the colonists when they saw how destructive the British Regulars were to the colonists property and how cruel they seemed for no reason. Most of these soldiers were young men who came from a farm not unlike the one the colonists were living on. This applied doubly to the way the British treated the large German-American populations in Pennsylvania and New York, where most of these companies were stationed. But do not think the German mercenary troops were much kinder, they partook in a huge amount of looting as was seen as a customary part of their compensation as soldiers. This did not endear them to the colonists either.

One thing they were absolutely known for was desertion. First off, deserters would probably have been executed if caught. Despite this, these units always suffered high desertion rates, simply because they were composed of draftees who were largely unmotivated by any sense of duty. In earlier conflicts in the Germanic states they would typically suffer 10% desertion rates. This was probably high because they knew they could return home fairly easily since they were on the same continent. The mercenary companies were also very poorly paid, and often paid late while in the colonies. By the late 1700, most of these Germanic princes who perpetuated the mercenary company trade were near bankruptcy.

Captured mercenaries also realized how great life was in the colonies. Working even at force labor for colonists they were earning more in pay than they did as a mercenary, and on top of that the colonists promised them free land if they deserted. The colonial government even started offering inducements for deserters. By 1778, the war was starting to take longer than expected for the British, and that is when desertion really picked up. By the 1780s units were deserting in the hundreds. Training and professionalism played no role in who deserted, only opportunity to do so. The elite Jager units saw the highest desertion rates in the war due to their independent operations and usually being stationed on borders. In the end about 5,000 stayed behind after the war.

The general assessment of the Hessian soldier in the American Revolution would be of professional yet unmotivated soldiers. They fought well and held ground when required, but often deserted at the first opportunity. As mercenaries they were already distrusted and vilified, and as a foreign person even more so.

Source: The Hessians by Rodney Atwood