I've written one of those before(1). For general browser traffic it isn't problem since you can add the proxy cert to the browser or whatever it uses. For apps (ie non-browser) it will depend on their certificate checking code, which for best practise should not accept any old gunk the OS/browser does.

(1) For a test I used my bank which had all sorts of RSA checked and similar logos all over the "secured" pages. Needless to say those logos remained even though I was going through a man in the middle.