I've spent most of my spiritual path wandering about as a solitary. While there are some who are forced to be solitary (lack of local groups, age restrictions, or whatever other reasons), for me it has always been by choice. I have belonged to groups in the past, ranging from the very small close knit group, the slightly larger informal group, and even a large very organized group. Still, I always find myself pulling away from group work, I simply prefer to work and worship alone. There are of course benefits to a solitary path (otherwise why make such a choice?), but it does have its potential drawbacks, as well. Interestingly, many of the benefits are linked to the drawbacks...
For example, one benefit I've found is the ability to work at your own pace and on your own schedule. You study what you want, for as long as you want, and practice on your own time. The downfall to this? You have to actually motivate and direct yourself. Sometimes you can find yourself stuck, unsure of which steps to take next. Or maybe lacking the motivation to follow through on goals you set. In a group setting you might have more guidance and support through such times, but as a solitary you must be your own support.
In a similar way, you are able to write your own rituals. Making them as long or as short as you want. Able to use only symbolism that speaks the most to you, to use the tools you prefer. The downfall? It's up to you to know what works for you, and what does not. It's also up to you to do all the work, both in preparing and actually in the ritual itself.
Another benefit is for the self conscious. When working alone you don't have to worry about having the eyes of others on you. You can say what you want, dance freely, sing, gesture, or whatever else without feeling self conscious. Of course, sometimes it's hard to get up and dance alone, or you may feel foolish talking with no one else around you.
There are just a few of the dual pros and cons. Like many things, being a solitary is what you make of it. What is beneficial to one person can easily weigh down another, if they are unable to use the benefits to their advantage. A strictly solitary practice isn't for everyone, just as a truly group based practice won't be best for everyone, either. We all have to find what system works the best for us.
For me? I've found my balance. The majority of my study and practice is done alone, this is most effective for me - but sometimes I celebrate with friends, and have a network of people (mostly other solitaries) to talk to, share ideas, struggles, and so on. Even solitaries who love being alone and working alone can benefit from the input of others from time to time, and we can all get a little lonely now and then.