I use 000webhost.com for about a dozen sites. They are all business related. I too buy my domains from 123-REG.
The system works well and like their blurb says, they are rarely off line. I would recommend both companies actually.
The downsides are few but if pushed I would say that uploading your content to 000webhost is slower than the paid hosts I have used. By slower, I don't mean it takes a long time once connected. I mean that connecting to the host sometimes takes a while, particularly at the weekends.
Another downside is that you have to make sure that you either are uploading regularly or you are getting visitors to the site regularly. If you cannot fulfill that criteria your site is likely to be put in the "for deleting" queue.
It isn't a big problem, so long as you are aware that it may happen. Visit the members area regularly and if you see that your site is marked "for deletion" simply click the "protect my site" button and it will never be deleted.
The final thing to mention is that it is a little challenging (to say the least) to figure out how to connect 000webhost to 123-REG. It is worth the effort though, as it will save you a fortune on hosting fees and nobody will ever know you are using a free hosting service.
If you or any other members get any problems, post a message on the forum and I or one of my colleagues will try and help - also for free haha !.
Broadband Upgrade Team