Air Conditioning Recharge, caveats?

[SiennaDriver]

2000 Sienna, 150k miles. The a/c is poor, almost non existent, unless the cars going at a good speed and even then it's minimal. System has never been serviced.... my guess is it simply needs a recharge.

Any caveats using an off the shelf recharge kit? It'll be my first time.

[Black Sunshine]

The kits are really easy to use. Pay the little extra for one with a gauge. I have a kit because I thought mine was low on refrigerant but it wasn't. I don't know much about AC. I know that if it needs to be recharged the refrigerant got out somewhere. Nobody let it out and resealed the system. The other annoying thing is that having full pressure on the refrigerant isn't everything. There are other components that can cause it not to work.

My Sienna has 227,000 miles and it's 11 years old so I'm just waiting to have an AC problem. I figured I would cross that bridge when I get there and figure it out then.