Use a lawyer only if your case is a difficult one, like when you need some legal background knowledge to be able to get thru, IE: a gay binational couple never married trying to immigrate as a couple and living in other countries or situations such as unsettle divorce or pending child custody. On the other hand if you have a plain simple background (only one marriage, all documented, work all documented, etc) then you would throw your money out the window because the immigration lawyer would do exectly what you could do yourself.
Also be on the lookout I can assure you there are more scammers than real lawyers out there
As to if its faster I dont think it makes it faster, because if you use a representative all correspondence between you and CIC will have to go thru the representative, so instead of sending docs to CIC directly you will have to send it to you lawyer for him to analyse and then send it to CIC, sometimes they take from days to weeks to do just that depending on how busy they are. At the end this could add 1 or 2 months of plain paperwork shifting.
The cost of canadian immigration lawyers in Canada runs from anything between 1000 to 7, 8000 dollars
good luck