In my two weeks in Sri Lanka my approach to the means of transport available in the country was a bit of mix’n’match, partly because I wanted to try different options (in general choosing the cheapest available) partly because on certain routes you have no choice!
So let me share here my experience and comments on all options I tried:
#Plane. Only to get into the country and back out again. The Bandaranaike airport is 35Km north of Colombo and can be reached by public transport. 
There is an airport bus that leaves from a small car park on the left as you exit the airport – if you ask the airport staff they will point you in the right direction.
Rs 200 is what I paid for a single trip to Colombo but the fare seems to vary, depending on your bargaining skills. Alternatively you can catch a taxi and pay extortionate prices.
#City buses. I like them so I used them most times, even when carrying my suitcase. Buses are very crowded and never stop for more than a couple of seconds so you often have to jump on/off when the bus is still moving. Not many Westerners semed to travel like this but I think that everyone should try. And the locals were always helpful (I was offered seats, given directions, etc). Bus fares are ultracheap, for example I paid Rs 15 for each few Km ride within Colombo. You buy the tickets on board and the ticket guy never has much change so make sure you have small notes and coins on you.
#Long distance buses. The “standard” ones look like city buses and have no a/c. They get very crowded and very hot. My journey from Colombo to Palavi – just under 4hrs – wasn’t overly pleasant in the tropical heat of a late morning/lunchtime but it only cost Rs 150 (less than £1!!!). I wouldn’t do it everyday but it’s an interesting experience. Just bring lots of water and don’t expect toilets on board or toilet breaks. Minivans with a/c are available on most routes and are a much more comfortable option, at only slightly higher fare (Rs 180 for Anuradhapura-Dambulla, Rs 200 for Dambulla-Kandy).
#Trains. I travelled 1st class between Kandy and Colombo, and 2nd class between Colombo and Galle (and back). 1st class had a/c and seat reservation, very comfortable although not comparable to a Western-style 1st class. I bought the ticket at the station two days before travelling and had no issues whatsoever.
The 2nd class train had no a/c and the seats were grubby but ok. No seat reservation (you buy the ticket on the day) so get on the train quick and sit by the window to get some breeze! Trains can get very crowded too, though, and while my journey from Colombo to Galle was fine, the return was a bit cattle class. Yet I cannot complain: I sat the whole time (ok ok, it was on the floor next to the toilet!), I had fresh air coming from the open door and made friends with some backpackers who were sharing the floor space with me. Much luckier than those people standing in the middle of the aisle!
#Tuk tuks. Also called “three-wheeler taxis”, they are good for short-to-medium distances, cheaper than standard taxis and the favourite choice of many locals and tourists. As for me, I used them only when there were no buses available. In Colombo tuk tuks have meters, everywhere else you’ll have to agree a price before starting your journey. My advice is: have an idea of what distance you are going to travel and haggle accordingly. In Colombo I shared a tuk tuk with a fellow traveller for Rs 50/Km, the standard rate. In the countryside I negotiated Rs 1,200 for a 17-Km ride.
#Private car with driver. Not something I would usually go for, as too expensive when you travel on your own and also it lacks the character of catching public transport. But I used it once as I joined a couple who was travelling in the same direction as me. The 2-hr journey from the Kalpitiya area to Anuradhapura cost us Rs 12,000 in total, superpricey compared to my travel standards, but in 36C heat it was a welcome change!
Overall I found it easy to travel around Sri Lanka by public transport but be prepared for slow journeys and not the maximum of comfort. If you are in a hurry or cannot do without comfort, choose the private car option. I will meet you at destination when I arrive by bus :-).
