Don't have them on rental, but have one on own house with proximity access built in.
Its great not having to fumble with keys in dark it's just a little tag that you have to get close. The codepad means I can even leave them home when going out and not using car.
The better ones have multiple codes. The "Master" code is required to change user codes. So you keep the master to your self and supply the tenant/agent with one or more codes.