I've been using a belly putter for 11 seasons now, since my second year of playing. I'd heard they were easier and I didn't have a lot invested in an old putting stroke. I had somehow bent my first putter and did not notice until I took a lesson and the pro said "you're putter's bent" like you could not putt well with a bent putter. When I made a 30 footer with a 6 foot break on the first try, he decided not to give me any putting tips. But I found myself at the end of a ski show a month later with some free time and a bunch of golf booths in the back. These guys that were just selling putters were starting to pack up, but were happy to help me out. I tried a couple of models on their little practice runway and wasn't impressed. The guy helping me watches me putt a couple, grabs a putter from the rack, makes a few tweaks here and there and then hands it to me. A couple tips (e.g. move the ball forward in the stance to directly under the left eyeball), and I proceed to make 10 ten footers in a row. End of show discount = 1/2 price? Sold! As much as I love my putter, no one who has ever borrowed it for a couple of test swings has ever said "wow!". Getting it fit for me is what really made this thing work.
What I learned#1 - Get fit for your putter!
I got a belly putter (vs a long one) because I walk a lot and the belly was long enough sticking out of the bag. It was pretty easy for me to adapt to a pendulum swing vs a screen door swing because I had no idea what kind of a putting swing I was using anyway. I quickly started using a claw grip, naturally without being taught to just because it seemed right. Separating the hands eliminates left wrist flipping. I did not anchor the putter end into my belly because I didn't know any better. Instead I anchor my left wrist to my belly. Strangely enough (as I understand it), this bypasses one of the purist digs against long putters (the argument that anchoring the club to your body violates a rule of golf). When I fool around with short putters, I still use my long putter technique. I just have to bend over a little more. A taller putting stance is more comfortable. I also believe a taller stance makes it easier to make a pendulum swing and see the target line.
What I learned#2 - Find your own stance, grip and swing that is comfortable.
My observation is that left eye dominance is a huge advantage for putters. When you can set up with your left eye directly over the ball, you not only see everything square to the target line, but you are also making contact with a slight upswing. I've seen a lot of right eye dominant putters line up out of square so that they can get their eye kind of behind the ball on the target line and set the ball up closer to their right eye to reduce the angle. The result is most often a trig test for aiming the ball and putt that starts with a hop bouncing out of its resting position instead of topspin rolling right from the start.
What I learned#3 - Some people are going to have a harder time putting no matter what kind of putter they are using
From all the putting instruction I've taken, read and watched - a pendulum swing is a simpler swing. You just rock your shoulders. Direction is a given if you line the ball alignment mark to your starting line and start with your putter square to that line. Distance is controlled by how far you rock (in my case back to the right side of my right foot is ten feet, 15 degree angle is 20 feet, 20 degree angle=30 feet - calibrated slightly more or less to green speed). Putts over 40 feet are more problematic with a long putter than a traditional one because each degree of arc increase introduces a logarithmically higher margin of error. A long putter makes it easier to make a pendulum swing.
What I learned#4 - Pendulum swing makes more putts for me. Making more putts inside of 30 feet more than makes up for any loss of strokes from over 40 feet away.
I play as a single a lot. I play with a lot of different people. I get asked a lot about my belly putter. I also get a lot of perplexed "is it you or is it the putter" stares on the green (this is just weird). And I hear a lot of comments (e.g. that's such a smooth stroke, I've never seen the ball roll like that, that's it I'm getting a belly putter). I still misread break and distance and miss short putts with the best of them. I still have good days and bad days. Based on the people I play with, I think I make more than my fair share of putts. Based on an average of 32 putts per round - my results are not spectacular. When you're fit for a putter, you can feel the difference in how much easier it is to putt. When you get consistent feedback from different sources, you know you're on to something.
What I learned#5 - I know the long putter works because ... regardless of whether it really does or not, it makes me "know" that it works.
Bottom line - if you want to improve your putting it's going to take time, money and/or effort. The more you put all 3 into the pot, the better your results will be. Regardless of the club and your technique, green reading is still half of the equation. There are lots of reasons why a belly putter alone won't do a damn thing for your putting. There are lots of reasons why a belly putter can be the beginning of a giant leap forward in your game. In my case, there are three good reasons why a belly putter has been a great investment:
1) Hearing "Son of a ... BITCH!!!" twice a round
2) Seeing the wheels turn in my golf buddy's head when the beers are on the line because he knows I can make any putt and he only has a snowball's chance at Cataloochee in July of making anything over 12 feet.
3) Watching the last 6 feet of a long putt dropping into the hole gives me a really good feeling