I'm always astounded when people tell me they had an awful make up artist do their wedding make up. Don't you check out their past work before choosing who to use? It's always a risk taking a blind punt, even if it is on someone else's recommendation. They might have different taste to you. If you do decide to get it done at a MAC counter you can request a particular artist rather than just getting who you're given. The risk there is that you won't be able to see any of their work before you choose but perhaps you can see who's make up you like in the store (and double check they did it themselves because often, when it's quiet, they do each other's!). Don't forget that you will get an array of experience and skill levels at a counter too. Some fresh out of make up school and some with years of experience who do high profile celebrity and fashion shoots.
If you do it yourself then practice your look plenty before the day and get a friend to take photos of you in various lights to see how it looks on camera.
Don't forget to bear context in mind when doing your make up. I'm not saying go mad, but your dress, hair, surroundings etc will be much more dramatic than your day to day look, so your make up should be too. Even if you want to keep it natural looking it should still be a lot more "special" than your normal natural make up - agreeing with thomasbodley's comment about Kate Middleton's make up, there was nothing "special" about her make up. It was fit for a shopping trip (and I mean the supermarket, not Bond Street!).
Apart from looking great and having your make up last all day long, another benefit of having a make up artist is that you get an hour of calm and relaxing time to yourself before the wedding which can be really helpful. And don't forget that you will be looking at your wedding photos for the rest of your life so in the grand scheme of things £150 isn't a huge amount of money to invest in making sure you look your best - just as long as you do the research and get someone who knows what they are doing.
Another hint to those disappointed by make up artists - Communication is extremely important. We are not mind readers! The only time anyone has ever been disappointed with something I've done for them was due to them having a strong idea in their head of what they want but failing to actually tell me. If you don't know how to explain what you want, collect pictures from magazines to take to your trial. If you don't like something at your trial tell the artist and they can change it. This is what a trial is for, after all! Don't just go off disgruntled.