An analyzer is nice for doublechecking the shop's analyzer, figuring out what's in a tank when the label falls off, and knowing what you are diving with when you have a tank with some old nitrox in it topped up with air. In the latter instance it is particularly handy since the shop may not have an analyzer you can use. Also, if you pick up a tank for someone else it is wise, for liability and other reasons, to have them analyze the tank themselves before using.
I can't image diving nitrox without one. While some shops are very free and easy about letting you use theirs, some make a big ritual out of the process, and it can just be a pain to try and use there's, expecially on those occasions when you want to analyze but haven't just bought a fill.
I wouldn't necessarily run out and buy one right away. Take you class, analyze a few bottles, and find out whether you are going to be diving nitrox on a regular basis.
An analyzer doesn't have to be all that expensive. You can build one for about $100, following the plans in our book, or using one of several kits based on those plans like the one Oxycheq sells. The homebuilts actually have some advantages over the cheaper commercial models.
