Pinyin is not enough, unless you never want to be able to read or write. Pinyin also gets very confusing as your vocabulary grows, and reading pinyin that has no tone marks is hell unless you are fluent, and even then it is hard. Many Chinese words, scrap that thousands of Chinese words have the same pinyin, tang2, tang2, tang2, tang2...do you know what tang I mean? Now lets look at those words again 糖，堂，塘，螳...now there is no confusion about what tang I mean. 

If you do not learn hanzi, you will never be able to read any native Chinese material, none of the Chinese languages are written in pinyin, and even though Chinese people learn pinyin as children, they are not exposed to it on a daily basis and they almost never use it. I have seen many Chinese speakers make mistakes when being forced to communicate in pinyin. 

It entirely depends on what your objectives are. I guess you know that Chinese writing system is based on characters, not on Pinyin. However, if you learn pinyin well and can speak Chinese, this already would be A LOT. Another question, if you will be able to do this. Learning pretty much any language requires reading. How are you going to read Chinese without knowing characters?

Characters are important. You may have trouble remembering vocabulary in pinyin only. Also you won't be able to read anyhting and you'll have to stick to spoken chinese.

But pinyin is also important, to help you learn the characters and use a computer.

I suggest learning both at the same time.

You need to practice Chinese pronunciation at first.

It's better to learn Chinese Character after enough pronunciation practicing.

If you just want to improve your colloquial Mandarin, then I guess it's totally ok to learn pinyin only since many people only learn Romanised arabic instead of Arabic inscript.

@Michelle

There are many Chinese languages used in mainland, it's impossible for every Han Chinese(汉族) to speak Standard Mandarin/Putonghua "correctly", because putonghua is nobody's mother language, everyone has different local mother dialect/language, just like I'm now in Shenzhen, Guangdong, I hear different tongues everyday, Sichuan dialects(四川话), Hakka(客家话), Teochew(潮汕话), Cantonese(粤语/白话), Gan language(赣语), Lei Min language(雷州话) and some from my hometown as well;

even in northern China where it's thought to be the origin of Putonghua, people still speak Putonghua with local accents, beijingers use "-er“ a lot and they also have dialectal words and phrases;

I don't really think anyone in China uses Putonghua as mother tongue untill now our "dear" government encourages every child to use this tongue, which has caused many children to speak Putonghua with dialectal accents because their parents or teachers don't really know how to speak this common tongue properly.