I actually like the mini-gauges for deco bottles. Less things to go wrong and they are nicely out of the way, well about as out of the way as you can get.
There are two types on the market the Genesis/Oceanic/Probably-someother-brand-as-well and the Apeks.
I have an oceanic branded one. My first one died within three months. This was replaced and has been going strong for around the 100 dives it has been used on over two years.
General consensus amongst my diving peers is that the Apeks is slightly better because we have never heard of one just breaking and it is slightly easier to read.
And, you can check how much gas you have got. Granted, not as accurately as a full sized gauge. In the big scheme of things this matters not on a deco bottle because if you cannot breathe through the reg, no matter what the cause, you have to go to whatever strategy you had as a backup anyway.
Go on, tell me I'm wrong! Dare you! ;o)