Zroam- ever heard of it?
 

zroam.com

How does it compare with other offerings? I'm interested in roaming in US. Also, does anyone know if callback services like Jajah work with these cards? Can I effectively call for free? JAJAH offers free calls from any mobile phone

The free incoming is only if you are being called on the UK mobile number. Calls to the US number are surcharged 25c/min. 46c/min outgoing for domestic calls in the US (or within the UK to a landline) is pricey.

Zroam, like many providers now, have a "fair use" policy (see page 4 of their terms and conditions) which essentially allows them to cut you off if you have an (unspecified) unfair proportion of incoming calls. So, as always, callback would work technically (if you can get a cheap rate to UK mobile), but in practice means the sim might be cut off at any time, without refund.