As a Nationwide customer, I would say avoid - there are cheaper offerings with better customer service out there.
I had a shocking example of service that ended with the branch manager calling me to apologise - not going into details as it concerns financial info I'm not going to air on a public forum, but my fixed ends in a year and I will not stay regardless of the deal they offer me.
I personally wouldn't fix - make sure you have the flexibility to fix in the future but with rates as low as they are you should have plenty of time to asses your options when rates rise, and enjoy lower payments in the meantime.
If you're on the ragged edge of borrowing at these rates, I would think very carefully about whether you're doing the right thing or should rasie more deposit.

