Learn to Speak Thai
When you come to Thailand, you'll soon realise that you're in a country who's language is an elegant sounding, complex mix of musical tones. The Thai language is not really an international currency, like French, English, Cantonese or Japanese but if you plan to live here long term, there is no doubt that being able to converse in the local language will enhance your enjoyment of this diverse society.
In reality, few Thais will expect a foreigner to be able to speak their language and they will usually be quite surprised if you start talking to them. There are many stories and anecdotes from expatriates, where a Thai national is talking about them openly, assuming they will not understand, often resulting in very funny or embarrassing situations when they finally realise that this foreigner can actually understand them.
That said, other than domestic travel within Thailand or bordering countries, speaking Thai is not particularly useful. Internationally, you'll probably only have the opportunity to use it in the Thai expat communities of other countries, or in overseas Thai restaurants. This leads us to the only other advantage I can think of... and that is the kudos factor. People will be most impressed at your command of a complex Asian language, including the Thai people themselves.
Having Thai language skills allows you greater freedom to explore remote areas and fully appreciate the diverse cultural idiosyncrasies of this beautiful country. If you stick to the tourist ghettos of Bangkok and Pattaya, then you can probably rely on the limited English skills of the Thais that work in the tourist areas... but if that is the case, you're probably the type of person who will never make it past the thin veneer of tourism and unlikely to experience anything other than the beer bars and Western cultural imports.
To be fair, there are many long term expats who simply don't have the time to learn Thai but get along just fine and with effort, still manage to enjoy the local flavour.
Should I Learn to Read and Write Thai?
Learning to read Thai is a long, often frustrating process. However, the rewards for perseverance are significant. Just imagine being able to read local newspapers, road signs, writing to your Thai friends and colleagues etc. If you don't learn to read and write, you will effectively be an illiterate.
There is also the argument that reading and writing skills will enhance your ability to speak the language, as you will have a better grasp of tones and linguistic structure. You will have a much better idea about phonetic structure. Looking up words in a Thai dictionary will also become much easier.
Consider this - If you make this country your long term home, sooner or later you will have to learn the language and you will probably wish you'd learned the alphabet right from the start.
Is it Difficult to Learn Thai?
Like other Asian languages, Thai is tonal. There are high, mid, low, rising and falling tones to contend with, all of which can make a single word mean something completely different... so if you tried to speak with a completely monotone voice, it would probably sound like gibberish to a Thai speaker.
There are also more characters in the alphabet. For example, one of them is half way between a 'B' and a 'P'. Failure to pronounce them correctly can completely change the meaning of the word.
On the bright side, the word order is quite simple. There are no genders or plurals in nouns and there are no definite articles. There are many other good points but they are too numerous to mention in this short article but believe me when I say you shouldn't let the complicated side of the language put you off learning.
In summary, it is considered to be an easier language to learn when compared to Chinese or Japanese because the Thai script is easier to learn. However, many struggle with the complicated rules about tones etc. If you throw yourself into the language and expose yourself to it on a daily basis, you'll pick up basic Thai quite quickly. Becoming fluent will take much longer.