Panasonic batteries work great in my Opti. One set lasted for three weeks last year in the Sierra. I did carry a spare set last year, this year I will not be carrying spare batteries. I purchased my batteries from MEC.CA in Canada.
The beauty of the Opti is it does not change the taste of your water. During my three weeks in the Sierra and trips in Canada last year I used no filter of any kind, worked great.
The Opti draws a very small current when not in use. During a trip I leave the batteries in, between trips I take out the batteries.
Ultra Violet filters have a reputation of being unreliable. Carry a backup system to process your water, I carry Chlorine Dioxide pills just in case the Opti fails. During my three weeks in the Sierra and two trips in Canada the Opti worked perfectly, and I did not need the backup. This year I will still carry Chlorine Dioxide just in case.
I keep the Opti in the foot of the sleeping bag if the temperature is expected to
go down to freezing.
I use the UV filter with a 1/2 Litre hard Nalgene bottle, and process 1/2L of water
at a time.
For me the Opti filter strongest feature is that water is processed very quickly, as well as being easy to use, and not changing the taste of the water. The Opti is a good piece of kit, just keep it clean and dry. It's flashlight mode saves having to carry a spare flashlight/headlamp.