There are many differences between working with a coven and working as a solitary. Whether these differences are good or bad is up to you to decide for yourself.
When working in a coven, you more people, and can, naturally, generate more energy then when you work alone. This can have drawbacks however. When you work with a coven, the energy you get depends of the type of energy the group puts out. Say that one coven member is in a really bad mood, but goes into circle anyway without letting go of their problem. Now, when you raise energy, you have that negative energy mixed in. This is not done on purpose of course, but it is something which can happen. When it is just you raising the energy, you know how you feel, and what kind of energies you will generate. You will know what time of day you have more energy to spare, and so on.
Also, working as a solitary gives you the freedom to do whatever you wish. Say you really really want to cast a love spell on someone. Many Wiccans do not agree with casting a love spell on a specific person, and you would be looked upon badly if you were to try to drag your coven into a spell of this sort. If you are a solitary, you only have to deal with yourself.
With a coven, you know when you will be getting together to celebrate or work magick, which makes scheduling work shifts and such pretty easy. However, it is not always easy to call together the whole group for any emergency which might come up. As a solitary, you can cast a circle and perform a ritual whenever and whereever you want to.
In a coven, you can always be sure that there will be someone for you to ask any questions you may have, and learn from other people's experiences, while as a solitary you are limited to other sources of info, like books and the web for example.
Most covens have a definite stucture of thier practices. What I mean by this is they perform the same ritual for the same holidays each year, and they honor the same gods all the time. As a solitary, you can change a ritual everytime you perform it, and change which dieties are honored. Also, as a solitary, you are not limited to the groups beliefs when creating a ritual or a spell. For example, say you are in a coven that, for whatever reason, does not acknowledge the Egyptian Gods. You have a need to do a spell involving cats, and you wish to incorporate the help of Bast, the Egyptian goddess, who is associated with cats. If you are a solitary, then you have the freedom of choosing what dieties you worship and honor.
Well, I could go on for days, but I think you get the main idea. If it sounds a little like I am partial to solitaries, that is because I have been practicing as a solitary sice I began. Only recently have I found a coven which I have been attending open circles with. There are definitely some excellent qualities about working with a coven that you just cannot get as a solitary, but the purpose of this site is to help out solitaries, so that is what I am focusing on.