If you have the usual 30t small ring on your current triple and the bike with the compact has the usual 34t inner ring, the gear ratio from any given cog on the same cassette will be about 13% higher. That will feel like being about two shifts higher, depending on the specific layout of your particular cassette. You'll (almost certainly) react by shifting down two gears. So, if you use the lowest two gears of your current setup, you'll miss them if you switch.
Bear in mind that I'm making a big assumption here - that you're sticking with the same cassette. Choosing a cassette with larger cogs for the lowest gears would give you back those ratios, probably at the cost of having slightly larger steps between gears. That drives some people nuts and others don't notice it.
Suppose you currently have a 12-25 cassette. Your current lowest ratio is 30/25, or 1.2. To get the same ratio, you'd need a cassette with a 28.3t largest cog. Obviously you can't have a third of a tooth, but 28 is close enough. 11-28 (and 12-28?) cassettes are readily available.
If you're already using a 27t or 28t largest cog, you need a mountain bike cassette to hold onto those low gears. While doable, IME the jumps on a MTB cassette feel pretty big on the road. Whether or not this is acceptable is pretty subjective. You need to decide that for yourself.
If you already have a MTB cassette, don't get a compact. :wink5:
As far as the upper ratio is concerned - if you have an old-school triple with a 52t big ring and you actually use 52/11, you need to learn to spin. :wink5: 50/11 is a higher gear than 52/12.
For me, triple vs. compact is about whether I get to have acceptably close gear spacing on my cassette and what the shift between the inner and outer ring is like. Is it f'ing huge? Do I have to do it constantly? I have a pretty rapid cadence, and can be relatively happy in the 34t ring for all flat and climbing efforts on my own at an aerobic pace. Riding with a fast group or doing intervals, I can be pretty happy in the 50t in the flats, but I tend to cross-chain if I'm solo and not trying to be fast. Some people with slower cadences find that their preferred flats gear ratio is right in the overlap between the 50t and 34t. Then they shift a lot and are very often cross-chained. Not so good. Even this is somewhat tunable by choosing a different cassette size that moves a preferred gear more fully into 50t or 34t chainring territory. I don't think that's a great way to choose a cassette, but I have the luxury of not having to.
My nicest road bike has a 50/39/30 crank and 12-27 cassette. I really like that setup. I recently acquired one with a 50/34 compact and a cassette with fewer teeth on the largest cog. For how I use it, it hasn't really bothered me, but I do run out of gears now and then. Finally, I have a 'cross bike that I just switched over to a 48/34 compact off another bike. It has a 11-32 9-speed MTB cassette. I don't have a lot of saddle time on it because of some other damage. It seems to have low enough gearing (it had better, it's a lower minimum ratio than my nice road bike) and if I'm on city streets I'm not bothered by the size of the jumps, but with the previous crank and on race courses, I noticed them some. I'm anticipating switching to an 11-28 cassette if I think I can get away with it. It wouldn't be as bad if it was 10-speed, though, so take my conclusion with a little bit of a grain of salt...
I think any crank that required me to use an MTB cassette to have gear ratios I wanted on the road is too big. So if you want my opinion about whether or not you'll be happy with this switch, here it is - if your favorite ratios are right in the middle of the overlap between chain rings or you'd need to put on a MTB cassette to have some gear ratios you like for climbing, don't do it. Otherwise, you'll be fine.
Jeeze... this is probably why people usually give the sheldonbrown link and tell posters to do their homework.
Hope one of my points gives you the missing piece that makes the decision easier.
