What is the long-term outlook after a kidney transplantation?

Good. Kidney transplantation is generally accepted as the best choice to replace kidney function. Transplantation from a living related donor generally lasts longer than from a deceased donor. You have to take medications to suppress your immune system from rejecting the kidney.
Better than dialysis. Txp kidneys have intrinsic quality that varies from excellent (live donor kidneys) to barely acceptable (kidneys from older deceased donor kidneys). To prolong function, protect it with well controlled blood pressure, glucose, and weight. Anti-rejection meds must be taken consistently. In these circumstances, 1/2 of live donor kidneys last 15 yrs. 1/2 of deceased donor kidneys last 10 yrs.
Mostly positive. A lot depends on the type of kidney transplant one receives. A deceased donor kidney tends to last approximately 7-10 years whereas a living donor kidney lasts about 15-20+ years assuming that the recipient does everything right. Transplant recipients do have a high risk of cardiovascular disease and need to minimize risk factors for CVD.