How much does a name change, given or surname, have an effect on ones personality, outlook or approach?

No effect on personality. Outlook and approach—completely depends on the individual. When I changed my name it didn’t change my outlook and approach. I took it in stride.

Yes it does make a difference.
NY Times Science
“Studies showed that children with odd names got worse grades and were less popular than other classmates in elementary school. In college they were more likely to flunk out or become “psychoneurotic.” Prospective bosses spurned their résumés. They were overrepresented among emotionally disturbed children and psychiatric patients.”

In the book Freakonomics: A Rogue Economist Explores the Hidden Side of Everything by University of Chicago economist Steven Levitt and NY Times journalist Stephen J. Dubner, Chapter 6: “The socioeconomic patterns of naming children” addresses this issue.
The authors use massively large data sets, e.g. California names lists in the tens of millions, and census information to extract and correlate the information. It does make a difference.
A girl named Une’qua or will most likely be the child of an unwed 16 year old with a similar name.
A Megan or Britteny will most likely have two parents in their 20’s married to each other. Levitt and Dubner even correct for socioeconomics and detail that effect.
Some may not like their conclusion but it is hard to argue with so much data.

I changed me first name when I was an adult. It did not change my personality or even my outlook on life.

I am much more comfortable with the name I chose rather than my given name.

I have gone by my nickname my entire life and this came about because I am Cruiser the 4th in my family so my folks pulled a nickname out of thin air. Odd part was and still is I have to go by my legal name on official and legal issue and did so in school too. So all my class mates knew me by my legal name and only my friends my few friends knew me by my nickname.

But looking at @worriedguy‘s answer and stats….my legal name might qualify me for the unusual name department but I guess I dodged that bullet as I did well in school and have finally hit my goals in life. I guess this is due to a good upbringing and my dad Cruiser the 3rd did well for himself too and was a good role model for me.

But overall everything seems so official when I have to employ my legal name in things I do in my life and my nick name allows for a more casual laid-back approach to things.

When I got married at 21 I changed to my ex-husband’s name. I thought it was cool to have a new name but it didn’t change my personality. When I divorced the shlub twenty some years later, I changed back to my maiden name. While that didn’t change my personality, it was a challenge to get used to people calling me a name that I had associated with my childhood rather than any of my adulthood. When professors called my name in class I felt like I was in third grade again. I have since gotten used to it, but that was an odd sensation indeed. I plan to change my name again when I get married again in March, it will be interesting to see what that does!

I think it depends on the individual’s personality, that is, on how he or she handles the way he or she is treated because of his or her name.

See this article about Winner and Loser Lane.

When I married the first time I took his name like usual. By the end of our marriage I hated baring that name. It was a constant reminder of pain. When I finally divorced him and married again, I took my new husband’s name. Difference is, I keep my maiden name assessable. When I write I use my maiden maiden name and I went through much of college using my maiden name. It does bug my husband but it’s my way of not loosing myself again to any one person or persons. I admit, I do loose myself in my kids though.

Simone fits me better.

I changed my name last July as part of the process of my sex-reasignment. I found it to be a very profound and quite a moving experience, in particular the part about actually choosing my new name, which was a process that itself took around two years to decide. I found it to be a great priviledge, to be able to choose my own name; something that very few people take advantage of in their lives. The actual day that the legal documents were signed was almost like a rite of passage, and while I don’t feel any different internally from the way I did before, it’s a wonderful feeling when people call me by my new name. On the rare occasions that someone slips and forgets, and uses my old name, it’s quite jarring and “wrong”.

I’m in the same boat as @downtide, but my name hasn’t been changed legally yet. When I’m with people and in situations where I can use my chosen name, I feel much more secure and content. If I have to fill out paperwork or sign legal documents and when my family calls me by my given name, I feel absolutely miserable, like I’m lying and being forced into a box. I can’t really think of anything that it doesn’t affect in my life.

@MacBean There are still some family members who don’t know about what I’m doing, or that I’ve changed my name. It was quite jarring, writing Christmas cards and signing some with my new name and some with the old one.