I've got my 2nd Implanon implant in. I removed the first one because of annoying bleeding patterns, but after 2.5 of the 3 years, and only because I wasn't having PIV sex at the time. Now pregnancy is more of a concern for me I looked at the options and was quite happy to have Implanon again :-) I haven't experienced any other side-effects.
The big issue that women have with it as far as I understand is the irregular bleeding patterns. You might not get any periods after the first few months, or you might spot an awful lot of the time. There are treatments for annoying bleeding patterns, and some of them are quite effective. It depends on what's available in your area. I'm currently taking a low dose of oestrogen to get my bleeding pattern under control on Implanon- the dose that I'm taking is as low as you can find in a combined hormonal contraceptive (2mg/day of oestradiol, equivalent to about 20mg of ethinyl oestradiol) but seems to be quite effective. I've also heard of women taking the combined oral contraceptive pill, and I believe that low doses of mifeprestone have been trialled with positive results.
Efficacy is great- very very few women who are on Implanon get pregnant, much lower than the pill. If I recall correctly, the chance of pregnancy in the first year post-insertion of Implanon is lower than the chance of
pregnancy the first year post-sterilisation. I've found that I'm not having to worry about my contraception- I'm able to just assume it's in place and not think about it. I haven't been concerned about the possibility of pregnancy either time I've had the implant.
If anyone has any questions, feel free to comment to this post (or in my journal) and I'll try to help :-)