Originally, I did not see why people would spend $25/year for Flickr Pro just to post pictures online. That was of course, before I began using Flickr originally as a Free user and discovered how powerful it was and how enjoyable and simple it was to share your pictures!
At first I decided I could try Flickr out at first and use a free account and see if it would be necessary for me to pay for the Pro account. However, because I had a free account I was limited to only uploading 100 Megabytes each month and because of that I would have to stagger the pictures that I upload to get the best pictures online and hold off on the lower-quality images until the next month. I also could only make 3 sets, which group pictures from similar events or similar subject matter. This was not too much of a problem until I got my new Cannon Powershot 1100IS, which takes much higher resolution, pictures to significantly larger, file sizes. These two factors were huge in my decision to get Flickr Pro. Sets were essential for me because I like to have everything organized and easy to find. I thought about if for a long time but, then I realized it was totally worthwhile for me with as many pictures I take and would like to share with my friends and family.
For anyone who is considering getting Flickr Pro I recommend trying the Free account first and see if you want the extra features and if you do then go ahead and buy Flickr Pro. For $25 /year you get unlimited picture storage which I think is an unbelievable value to you. Flickr is great for casual photographers and if you are a more serious photographer, then you are probably better off with paying for Flickr Pro!