I don't think anyone can really say for sure whether a hiring a lawyer is better than not hiring a lawyer. A lot depends on the person and the situation. The application process is designed so that anyone can do it, but some people -- for instance, people with more money than time -- might prefer to use a lawyer. Or, people who aren't strong on details, documents, etc. A good lawyer can be a good resource in certain cases. I know a few people that used a lawyer, and got good results -- but they were the kind of people who preferred not to do that sort of thing by themselves.
In my case, I hired an immigration consultant. I completed the application myself but asked her to review it. She charged me $700 and, although she said my application was good, she gave me a list of pointers. Most importantly, she pointed out that we had completely forgotten to include one document -- an oversight that could have cost us several months of delays. She also gave a bunch of other suggestions and fixed a number of little errors.
I also believe there is such a thing as a bad application and a good application, and I suspect (although cannot prove) that good applications stand a better chance of speedy approval over good applications. I'm sure many applications are submitted with little mistakes, big mistakes, missing material, too much or too little relationship evidence, the wrong kind of relationship evidence, etc. etc. There are lots of people that can do good applications on their own, but also lots of people who will benefit from at least a cold-eyes review.
