I have my own small business and an accountant to do the books. They also do my self assessment for an additional fee of around £120. Being as I had a sizable rebate last time, I believe it is money well spent.
If your own SA is not particularly straightforward, I recommend getting someone to do it for you.
On the other hand, if it is a simple form filling because you don't have additional income, do it yourself as it is not that hard (particularly if you kept copies of your previous SA - most of this year's will be similar to last).