Sri Lanka is a very hilly island; because of this there isn't really a straight road anywhere outside of the cities. The highway system is being improved all of the time, but that seems to focus on the roads along the coast in and out of Colombo. If you want to venture into the interior roads are OK, but can be a little scary.
The three best ways to get around Sri Lanka are either by train, car, or bus. The trains are a great way to get around; they're fairly inexpensive, fairly clean, and fairly reliable. Some of the train routes are also very beautiful. The buses are very cheap, always overfilled, and some are rather sketchy. The only way I can vouch for the safety of the buses is to say that I rode several and I'm not dead, you may not however be so lucky. The buses often play very loud disco music, and fancier ones are sometimes decorated like disco techs as well. If you're riding a bus or train, and you're not getting on at the first stop, be prepared to stand no matter how long your journey might be.
Cars can vary depending on where you rent them. Drivers often come with the cars and they too can vary depending on where you get them. Hotels and guesthouses will often have driving services they can recommend, but be aware that you'll be paying a great deal for this service. A car and driver can cost hundreds of dollars where a bus and train will cost you less than ten. However if comfort is important to you, than it may be the only alternative.