Re: SMARTY
SMARTY doesn't offer the same "roam like home" arrangement that Three does in many other countries around the world beyond the EU, which is very useful and could be a deal-breaker for some.
(I briefly ported to SMARTY and then later back to Three again as Three's webshite wouldn't let me upgrade to a (much) better value "new customer" tariff (yeah, way to reward long-term loyalty, guys). The Three website could really do with a full rebuild, it's clunky as hell and not entirely mobile-friendly (duh).)
(Remember, if you are fed up with your mobile network and are out of contract, you can now port hassle-free without having to endure helpline queues and drones, just by texting: PAC 65075.)