All a visa service does is fill out the form for you (by asking you the questions) and tell you the documents required for your application. And if you read the article Rebbecajo linked to, you'll see that that particular company is a bad apple.
The application is quite straight forward. Hundreds of active members on this site have completed it on their own successfully and the success rate for applications from the US is about 98%.
Yes, I imagine it is frustrating that you have to go through this process when you have been married for such a long time, but the UKBA has no sensitivity to this and you have to follow the same process.
The thing we see a lot on this forum is people who have been married for a long time, seem to think they don't need to prove their relationship. You still need to include your marriage certificate and proof that you have been living together during the duration of your marriage. As long as your wife has held a job in the US making at least £18,600 in the last twelve months and has a job contract for a position making at least £18,600 beginning within three months of your application (or you two have £62,500 in cash savings for the past six months), you'll be successful. You have a house so you already have accommodation sorted.
Of course if you are more comfortable with a visa service or a solicitor, then follow your gut and do it. But if you want to give it a go on your own, you can post questions and a list of the documents you are submitting on this forum and you'll receive feedback from many people who have been through this exact same application.
Best of luck!
