I've recently setup 55g with CO2 as well. I went nuts, and splurged for:
COMPLETE ULTIMATE Co2 SYSTEM (best of the best)($600+ VALUE ! )
I have to say - I'm not very happy with the regulator that comes with that package, nor the internal co2 reactor that the base model comes with.
The regulator maxes out at about 19psi, which is just barely enough to reach the cracking pressure needed the up aqua inline co2 reactor that RiverCats mentions (which I also have been using on my return line from my Fluval 306).
Other than those two complaints, the Milwaukee sensor is great - same one mentioned by Hawkins, above.
I have a drop-checker as well - though I went with the mini drop-checker - and I would not recommend that either. Go with a normal sized one - it's very hard to see the color or to add the fluid and get the mix right in the mini ones - the stem is just incredibly narrow, and the bulb is tiny, so very hard to judge amounts.
I found setting it all up was pretty easy. The hardest part for me has been figuring out the efficacy of the whole thing. When my co2 is on, it turns my whole tank in to what looks like a soft-drink! There are billions of micro bubbles in the water. But they don't seem to get dissolved by the water in the tank all that well, and I have my doubts as to how well the co2 is at getting to my plants. So I'm not sure if it's the Up Aqua inline diffuser, or the low pressure limit of the regulator, or some other factor. I'm thinking of getting a larger reactor like the one my LFS uses, to see if that makes a difference.
Overall I'm glad I went co2 - but I might have saved a lot of money by skipping the regulator and just using timers & a drop checker to monitor the PH. It does offer peace of mind - I'm not going to ever suffocate my fish since it will auto-turn off if the PH did drop too low.