Another vote for Gandi but to answer your question specifically....
Gandi are widely regarded as being the most ethical registrar out there so if behaviour to date is a good indicator of future trust then their reputation speaks volumes.
No hidden costs. They do tend to be a pound/dollar or so more expensive, however that leads me onto...
There's horrible companies out there who engage in question practices so it really does make a difference who you go with
To give an example here's some of my experiences with 1&1 which might help you understand the sort of difference a good registrar makes....
- - Incessant cold calling with vile pre-written scripts designed to prey on un-technical people's ignorance (this was almost once a week at one point and took several attempt to get them to remove me from their list....)
- - Hidden transfer costs on various domains
- - 'anti-pattern' upselling of other services on their site
- - Weird billing practices (such as billing you months away from when the domain renewal actually happens)
- - At least weekly email spam that's relatively hard to unsubscribe from.
- - You don't so much register a domain with them as you have 'an account'.
To go into that last one in a little more depth it means that they insist on auto renewing your domain and will charge you to transfer a domain away so it's often easier just to pay.
Eventually my CC details with them expired and I decided to just let the domain go rather than continuing giving them money.
However in response they started sending me ever more threatening emails about my CC card being expired and then sending letters to my house saying they'd pass the debt for auto renewing my domain on to a collection agency!
What's worse tho is the domain name didn't need renewing for 8 or 9 months but they had reregistered it anyway just to try and make me pay even tho they knew my CC details were expired!!
Ps. No affiliation to Gandi, just a fan that thinks they've proved themselves as a company and deserve your money. Also think Namecheap are pretty good, though Gandi would be the one i'd recommend first and foremost, particularly amongst slashdotters!