I've used Nike+ since it was launched and nearly reached 2,000 miles training with the kit. I run with Brooks Addiction shoes so need a way of holding the Nike Sensor. I use and recommend Marware Sportsuit Sensor + Case for Nike + iPod Sport Kit. This is around £7. Shows no sign of wear so good value.
After buying the kit Nike + iPod Sport Kit I needed a new sensor after about a year. There is no way of replacing the battery - I had to buy a complete new set, but you can now buy just the Sensor.
I've also tried the latest Garmin and will post a review soon and my conclusions.