Think carefully about what career you want at the end; I was told that a PhD can make you look intelligent on a CV, but it isn't worth three years experience in industry (excepting jobs in industry that require a PhD). Depending on the sector, a PhD can be fairly useless and, unless you have personal reasons for wanting to do a PhD, employers will ignore it - they will be looking for real experience.
Talk to a careers advisor about the sector you're interested in.
Saying that, if you want to do it then go ahead - just be prepared that in many areas of work you'll come and be in the same position as people with undergrad degrees or Masters - it really depends on what you want to do