Our travels took us from Zagreb to Plitvice, but the lakes are accessible from all main cities (Split, Zadar, Dubrovnik). If you are considering travelling from Zagreb, make sure you give yourself plenty of time to buy your tickets and find your bus, Zagreb main bus station is a large place with many terminals and purchasing tickets can be a long and time consuming affair.
As with all buses in the Balkans/ Eastern Europe, make sure you have loose change to cover the fee to stow your bags underneath bus. This is usually a 1 Euro or the Currency equivalent.
The journey takes about 2:30 hours and is relatively uneventful, the only real sights are the forests when you start approaching Plitvice. Upon arrival the coach drops you right outside the entrance to the park and park hotels.
To stay in one of the park hotels is more expensive than those around the park, but you are paying for location (which is essential for an early start) and includes the re-validation of your ticket the morning after your stay, effectively giving you two days for your day ticket.
The best advice I can give anyone when visiting Plitvice, is start early, as early as you can. Make sure you're waiting at the gate before even the ticket operators get there. Walking the lakes is a tranquil experience and can not be enjoyed if you are surrounded by 10 tour bus loads of tourists and school children. The park opens at around 7:30, but if you arrive earlier than this and there is someone at the gate they will let you in.
The park itself is series of crystal blue lakes connected by gushing waterfalls surrounded by lush green forest. Trails are wooden walkways built through the forest and around the perimeter of the the lakes.
There are several routes of varying distances and time lengths, all begin and finish at the same place so you will always end up back at the entrance. The shorter routes include a bus ride or two and miss out a lot of the lakes.
A word of warning when walking the lakes during the summer and peak tourist season, watch your footing on the narrower wooden trails directly adjacent to the lakes. We arrived well rehearsed in the fine art of the English queue, but other nationalities obviously didn't get the memo. The tour groups operate a pack mentality and do not appreciate the idea of two way traffic and large groups will walk side by side, not giving an inch if you are walking in the opposite direction. Nor will they show any mercy when you are deemed to be in their way. One wrong step and a nudge from an over eager tour member, and you're getting wet at the very least, at worst you can wave good bye to that expensive camera.
If you are on holiday, travelling through or on long weekend break in Croatia, Plitvice is a must see and with convenient and reliable public transport there is no excuse not to go.
Continuing down south to Split was easy enough, there are good bus links and 5 minutes before the bus is due to arrive a minivan will arrive offering to take 2 hours off of your journey for a small increase on the bus price. The bus would be a more relaxing and spacious journey, but who can argue with a 5 hour journey being cut to 3, especially after an all day hike.
Good Resources for the park are:
UNESCO - http://whc.unesco.org/en/list/98
Official Website - http://en.np-plitvicka-jezera.hr/
Zagreb Bus Station - http://www.akz.hr/default.aspx?id=260