For a few different reasons, I think that it's fair to say that the free service on Browsershots.org is good for smaller jobs, but for larger jobs the free service has some true drawbacks. However, if you have a single webpage, and you only need to see how it looks in five browsers, then you are likely to be fully satisfied by the results of the free service.
When working on big jobs, some issues come up which show the free service to be less worthwhile. As an unregistered user, you are only allowed about 100 shots in a 24-hour period. If you need to see how your site looks in 25 different browsers, and you need to view three different pages, those 100 images get used up in a very short amount of time. Oftentimes you will find that you need to make small changes to your HTML or CSS. Once the changes are made, you need new shots to make sure that the changes work in all browsers. Soon, you've hit the limit, and you're stuck, unable to get more shots to continue working.
The good news is that you can register with the site, and get about 200 shots per day. But even this limit is easily reachable in 2-3 hours, if you are focusing intensively on improving a site's browser compatibility.
Another possible issue with the free service is the amount of time that it sometimes takes to get your shots. There can be times where the site takes as much as 45 minutes to send you your shots, which for many people is too long to be worthwhile.
Overall, the free service is very robust, providing a chance to see your site in any browser whatsoever. It's a great way to test out the service and see how it works, and it does work really, really well. However, in the long run it may be worthwhile to sign up for the paying service at Browsershots, Priority Processing.