Hey lustylou,
I currently have the implant, and I love it. Yes, there are side effects, but I find they're worth it and manageable.
The first few months I had it I had some occasional spotting, but I havent had any blood, period or spotting in months and months now!
I have heard accounts of people putting on weight with the depo and was told it could happen with my implant too. I was a 12-14 before I had it. Went up to just a 14 afterwards (I dont have the best diet though), but it seems to be coming off again now.
The only bad thing I find is that I can get period pains any time of the month now, but I'm still WAAAY better off than I was before. A few minutes of feeling slightly ick once or twice a week is preferable to one day a month where I felt like I wanted to die.
On the plus side it's there and it does the job. With the coil I'm fairly sure that you have to keep checking its in place. I'm sure it's a rare occurrence, but I've heard examples of it sliding about and perferating the inside of the womb, and that would make me paranoid. Even if not, I'd be worried I'd get pregnant if I accidentally moved it!
The implant on the other hand, just sits under your skin and does the job. As long as you get it replaced in time, there's nothing to worry about, and nothing you can really do to prevent it from working.
I'd definitely recommend the implant to anyone.
When I went to the GUM clinic and was told about my options, the doctor told me the coil was more what they give to older people. I'm not sure why that is, but it might be worth discussing with your doctor/nurse if you could get the implant, or what other options are available to you. I did alot of reading into contraception back along and did see that the injection should only be used for a certain amount of years before choosing another method. It's a shame that you have to change your method since you like this one, but it's for your own health. Still, you have the right to choose another method that you like, not be forced into an option you dont want, and it sounds like you're having doubts about the coil.
You should be able to talk to your doctor/nurse about anything. If you dont though, I'd highly recommend going to a GUM clinic. Thats what I did. I was shitting myself beforehand, I felt so scared, but they were so nice there I feel stupid now for feeling that way!