Lonely Planet Poland like all Lonely Planet guides is of course out of date when it comes to prices, opening times and so forth. Name one Lonely Planet guide that isn't. What the guide does do though is tell you what you will find in many cities, regions and towns throughout Poland. Many you will not be familiar with and you can read up on each to see if you do want to include it in your planning. Like all Lonely Planet guides, there are not many photographs so you'll want to do a Google image search of castles, towns and so forth to know if it's worthwhile going there or not. As always the hostel listings are pretty slim compared to what you'll actually find on the ground. At least these days you can just look up the Hostelling International, Hostelworld, BUG and other websites to easily find out firstly if there is a hostel where you want to go and to also read recent reviews of what each hostel is like. Of course you can also book them too which is a lot more convenient than when I first went to Poland in 2001 and you had to ring up each hostel and hope you got someone who spoke English.
Poland is a pretty easy country to travel around. More and more people speak English now. Not quite to the level of say Germany, Switzerland, Austria, Scandinavian and some other countries who have been in the tourism game for a lot longer but this adds to the fun. Do you really need one of these to take with you, no you don't, and you'll probably find heaps of other backpackers with one of these anyway in each hostel you visit. Plus there is also heaps of free stuff in hostels on the major cities, however the smaller or less touristy destinations you'll still want to look in something like this for info. But it nice to have (or borrow from your library) to help plan your trip before going.