I have 4 tattoos...and here is my experience:
~When asking people if they regret their tattoo, or the placement, or the design-ask people who have had their tattoos for 5 years or more.

The first year or two of a tattoo is still the "honeymoon" period.
I got my first tattoo when I was 17 years old. I will be 33 in a few weeks, so my first tattoo is almost 16 years old, I have had it almost half of my life. I SOOOOO hate it now. SOOOO hate it. Luckily, I was smart enough to get it in a place that is easily hidden by almost all of my clothing-on my upper thigh. I don't like where it is now, I don't like the design, I just simply don't like it, and it is there forever.
~Do tattoos stretch or change with weight gain/loss?
Yes, they can, and they do. However, it is going to depend on where the tattoo is, how much weight you gain and lose and how quickly, and where you tend to store your fat. I have a small tattoo (my 2nd one) on my belly, and this was done when I was 21. At that time in particular, it was paired with a belly button piercing on the same day. I thought I was hot stuff. However....now I am 33 years old, and have had three children. I am also a belly dancer, and when I got those things done, I didn't know that was what the future would hold for me. I HATE the tiny scar the piercing left in my naval. Hate it. I also hate the tattoo...and I plan on getting it recovered with something else to camoflauge it. The tattoo I have there now is too "cutesy" and I hate it. I have had it for 12 years. The tattoo has actually gotten a bit bigger, from being stretched out from 3 pregnancies. It went from the size of a quarter to the size of a 50 cent piece.
~Other information:
I don't hate ALL of my tattoos, just the first two. I love my 3rd and 4th ones. My third is over 10 years old, and my most recent one is about 5 years old.
My advice if you decide to get inked:
~Make it in a place where clothing can hide it. I don't recommend the neck, the lower arm/wrist, the ankle, or places where it isn't easily hidden. Even on the ankle, you should think about how you will feel when you are 70 years old. Do you really want the Winnie the Pooh on your ankle showing when you are a senior citizen wearing a dress or skirt?

Many younger people think that they don't care if it shows, but 20 or 30 more years of life experience CAN and often DO change that.
~Don't get a generic tattoo. What I mean is...something that 10 million people already have. Like a tribal arm band on a guy...or the butterfly on the ankle or lower back of a woman. No Tazmanian devil, either.
~No names of boyfriends/girlfriends/spouses. Ever. The minute you do that, you are dooming the relationship to fail. Remember when Johnny Depp got the tattoo "Winona Forever"?
As I said before, I don't hate my last 2 tattoos...but, if I could do it all over again, and have a "clean slate" I would. Especially given my profession...