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I am thinking of buying a Canon 100mm macro lens.

I wonder if a ring flash is a must.  Any experiences and suggested models?  I am browsing B&H site now and I see a Canon model (MR-14EX) with excellent reviews, but it's expensive.

Regards,
Adelaide

With the 100mm macro, you don't need a ring flash: a speedlite with a diffuser works quite well for me.

I've also purchased the Canon MP-E 65mm and the focus distance is so small here that a speedite wouldn't work. I've purchased a macro flash, but not a ring flash: I've chosen rather the twin lite MR-24EX which give you more control on the light intensity and direction. But it's even more expensive.

I would say a ring flash is a luxury item, not a necessity.  If you are shooting bugs and things outdoors, you might not need a flash at all.  Most of my macro work hasn't used a flash. 

I have been using a MR-24EX for 5 years now and it is very good.  Whether you need it or not depends on how close you get to your subject.  If you do butterflies for instance, you need distance with the 100mm, and any flash will do.  However, if you want to photograph something REALLY small, like a baby ladybug, and you want it to fill the frame, then you have to get closer and the MR-24EX comes in very handy for even lighting.  It's a powerful flash, and it can give enough light for max. depth of field at f22 or so.

Adelaide
Don't laugh at my home made plan,but it's cheap and it works!!I have a 60mm canon macro and for lighting I use......
Take a piece of white cardboard(thick ripple cardboard works best),cut it into a rectangle about 18X12cm.Cut out a ring in the bottom part just so that it fits tightly over your lens.Then use your pop-up flash.The white part that stands up in front of the flash diffuses and scatters the light so much that you don't get shadows or anything.
The i00mm is longer than the 60mm so maybe you would need to cut it a tad bigger.I sometimes even use a bigger piece with the 430EX speedlite ,but there's not much difference between the pop-up and the speedlite.
I've done quite a lot of test shots with a ring flash and my cheapo cardboard and the results are pretty much the same.
It's cheap and when the cardboard gets tattered you just cut a new piece.

Daleen

I use an inflatable diffuser... something lite that  http://www.amazon.com/Opteka-Inflatable-Universal-Diffuser-External/dp/B001GRU1AA/ref=pd_bbs_sr_1?ie=UTF8&s=electronics&qid=1236586991&sr=8-1, but other types of diffuser would works provided that the front face is close enough from the edge of the lens. Otherwise the lens would cast a shadow over your subject if the focus distance is too close.

Concerning the flash, I've now a 580EX, but I previously use an older 420EX: any speedlite is powerful enough to light a so close and so small subject 

Yea, the 100mm is awesome. My last few hundreds of portraits are most of them done with this lens. For macro work is great on a FF. On APS-C it's a bit too long for my likes (I preffered the 60mm).
I agree with those who say that on macro level you can use a normal external flash with some diffuser. Any 430ex or 580 will do great. And also fill flash for portraits.
I'd go for dedicated macro flash only on a mp-e 65 (which i'll buy this year ).

Here's a DIY ring flash. A bit much for my kit.  http://www.fredmiranda.com/forum/topic/658257/

I carry a small white umbrella and slave strobe in the car, which can be mounted on a tripod. I suppose that a bit much for hand held. Also bought a cheap clip on flash diffuser for the on camera flash on eBay. It slips into the hot shoe and holds a diffuser in front of the pop-up flash.

Here's another one, small foam tube inside the lens shade, flash pointing down into it. http://www.fredmiranda.com/forum/topic/657587/0?keyword=ring,flash#5856042 I think I need to make a version of this one. Looks like fun.

This one is very interesting, using fiber optics to transmit the light from a pop-up flash. http://www.fring.we.bs/

Saw a guy last Saturday that cut the bottom 2/3rds off a common translucent (or white would work) rubbing alcohol bottle. I liked it. http://www.diyphotography.net/homestudio/blz/soft-flash-diffuser

Easiest, most compact way to accomplish this is a piece of thin cloth with a rubber band around it, over the flash head.  If you don't have an old handkerchief to cut up, go to a fabric store and buy the smallest piece of some white silk, might be one yard. (or synthetic fake white silk) You have a lifetime supply. Sheets and pillow cases are too think. Find something very sheer.

This brings back days of medical photography.

General rule of thumbs less than 1:1 magnification one or two lights on the camera very close to the lens.  Do not filter the lights you need the contrast.

General rule of thumbs more than 1:1 magnification a ring light is a must because the lens is blocking the light. Tips:  filter up to half of the ringlight to give a increase lighting ratio.

Test, Test and test again. Macro is difficult and very demanding photography. Good Luck!