This is 2G, so I wouldn't use this in something that needs a very long time horizon. Carriers are beginning to announce end of life on their 2G services. (For a hack it might be useful, provided you don't mind replacing it in a few years.)

Additionally there are security issues in using 2G/GSM, for example the user does not authenticate the network.

http://www.zdnet.com/article/hacker-demos-gsm-call-hijack-ca...

That depends on where you are. In the UK, there are still lots of rural areas that only have 2G coverage. The networks here won't be switching off 2G any time soon.

Also, it looks like they are selling a 3G version anyway...

Three have never had 2G coverage, so it depends on your network.

Yes.. They won't switch off what they never had. Three use someone else's (Vodafone?) and they won't be switching that off anytime soon.

Three used to use Vodafone for 2G but since they've merged with O2 I believe they use their 2G network infrastructure now.

Actually, 2G is seeing a lot of growth in low-data-rate services. Providers are talking about switching off 3G instead of 2G. This 2014 article (in Dutch, sorry) states that there are still new 2G-only devices being made: http://tweakers.net/reviews/3453/3/mobiele-netwerken-het-twe...