As an American, married to Canadian, going through this same process, here are my thoughts...
If you have the extra cash, a lawyer will provide peace of mind. However, and other lawyers may differ, the only they they really did was gather all the forms together in one place for us and mail them off. Now, that may not be worth several thousand dollars to most people, but at the time, it was a huge relief. In addition to gathering the forms, they were very good about informing us when new forms were issued by the CIC and making sure we did not fall into the dreaded "old August form" trap that so many others did. It was also pretty sweet knowing we could email them at any time to ask questions about the process.
However, ultimately, if I had it to do over again, I would have saved that money and gone through it all myself. The other posters are correct. While it IS super intimidating at first, if you take your time and really read all the material, it's pretty self explanatory.
Lastly, as I'm sure you've seen, the treasure trove of knowledge and experience on this site is amazing. While I have seen some incredibly bad advice given by some (which you'll recognize by the fact that everyone will immediately point out that it's bad info), the majority of information here is spot on.