Many camera phone accessories simply don't make any sense.
A cell phone camera lens might be an exception to this statement. A specialty mobile phone lens may be a worthwhile accessory for you for several reasons.
The three main types of lenses are macro, telephoto and wide-angle with fish-eye lenses being included in the wide-angle category.
A macro lens attachment will simply allow you to get much closer to your subject, sometimes closer then a distance of one inch. Macro photography can be a lot of fun and literally opens a new world of possibilities for photography with a camera phone.
Macro photography brings with it a very small depth of field to work with so getting your subject in clear focus can be a challenge.
Also remember that when you are in quite close, even the slightest camera or subject movement is exaggerated and you can end up with blurry pictures. Tip: get as much light on your subject as you can and your macro photos will come out much sharper.
For some mysterious reason the telephoto lens is the least popular cell phone camera lens attachment in terms of the number of them purchased.
The fish eye lens can give some really cool photos, but like the other two types, takes a bit of experimenting and practice to get some good images.
One of the nice characteristics with a fish eye lens is that everything is in focus. Fish eye lenses have a very deep depth of field, meaning everything close to and distant from your camera is within the range of sharp focus.
If you have a fairly high megapixel sensor and some decent lighting, you've got a good chance for a great photo.
Realize that the axiom that you get what you pay for is very true for just about piece of photography equipment you can buy. What you must remember is that with cell phone photography you already are working with a quality that is less than that of a point and shoot or digital SLR camera to begin with.
Think it through before buying a cell phone lens. If you think it will be too awkward to use, you probably won't. Like any gadget, it should be at least somewhat easy to attach and should not be difficult to carry.
Some systems have a clamping apparatus to attach the lens to your mobile phone. This is more secure then some of the more suspect adhesive-magnetic attachments that may not be as functional or durable.
Have a blast!