I use a Crate Taxi TX15. Weighs 13 pounds and is a bit larger than a can of paint. The sound ranges from very good to pretty bad, mostly depending on the condition of its internal, sealed lead acid battery. A new battery will have loads of back power that will hold strong for loud transients (This is called 'stiff' power)and, as the battery suffers abuse, that character fades. The amp sounds pretty great for that first week after a new battery goes in.

I've recently learned how to best care for an SLA battery and thought I'd share what I now know:
The two most important things to know are to never, ever let your battery get low enough that your signal sounds distorted. At the first sign of distortion, pack up the rig and start charging it. Second, never overcharge it. A full charge takes around 8hrs. 24 hrs for a 'battery has been sitting around unused for a month' and, charging back from distorted sound should take around 4 hours.

Best practice is to 'top up' your battery, as in play for two hours, charge for an hour. Batteries like to be recharged as soon as possible after run down. Batteries hate to be stored in a discharged state.

If your amp has been sitting for just a day or two, charge it for an hour before you leave to busk.

Replacing the battery in a TX15 is a very simple operation as well and the replacement currently costs $18US shipped on Ebay. Models directly interchangeably sized are: The OEM is a UB1272 (12v 7.2AH) and the upgrade is a UB1280 (12v 8ah). Both need to be ordered with 'F1' or 2.1mm connectors.