Thailand is best explored on your own. There is so many things to do there, going by yourself will help you choose activities you would like to do. Also, all the palces are easily accessible and information is readily available.
Better go by yourself!! most tour operators will definitely charge a premium of atleast 10% to 15% above the actual cost and hence to cut cost plan a trip yourself. More over tour packages have restrictions regarding timing as well as places if it is a group tour. If you are travelling solo or with a group of friends stay at hostel, use public transport and like wise you can meet new faces and also can explore the place in a better way and also save a lot of money which you can otherwise use for trying different cusines or shopping.
1) The thing is when you research and book every thing on your own that is when you learn.. there are many aspects and things learned while planning a trip which you may miss out if you ask a travel agency to book.. 2) You save on cost by booking trips by yourself.. Spend that save money to explore some additional places in Thailand or keep it for your next trip...
3) You see places based on your own wish.. No rush.. no one to tell you go here go there.. if you like a particular place you spent more time there but that is not possible if you travelling thru an travel company.. Explore the world thru our own eyes and not thru the travel companies...
Travel on your own. Little bit of research and little bit of patience
1) You can change your itinerary as you want and explore new things in place always
2) New Experiences are learnt if a plan flops
3) No pressure to complete your visit, you can spend any number of hours in single spot(adding you can be lazy and even wake up late as per your need)
4) Makes you a better planner and improves your confidence
If its your first international trip you may take help of some travel advisor. However if you have travelled earlier you can easily plan a trip to Thailand on your own. sufficient information is available on internet to help you plan the trip
It's always better to plan your trip yourself and get going. You can take the help of various packages available online to plan your itenary. Going on your own gives you more flexibility apart from being economically better!!