Summary: "Rio" is just fine."Ryo" is a common name in Japan. It is mostly for men, but there are some women, too.
When I first came to US, I was a little surprised to find almost all native English speakers cannot pronounce my name correctly, even after I told my name to them again and again. But on another thought, it is quite natural, since the sound of "r" in "Ryo" is not really the sound of "r". It is rather closer to "l" or even to "d", as some website explains. Here is a link to mp3 of the correct pronounciation of "ryo". It seems like a term in "Go", which I'm not very familiar with.
Most common mistake is to read "Ryo" as "rai-oh". Possibly from similarity to "Ryan", people read "Ry" as "rai" and add "o" at the end. In fact, "ryo" should be pronounced in one syllable, a consonant "ry" accompanied by a vowel "o". Just in case you're interested, there are only five vowels in Japanese ("a", "i", "u", "e", "o") and a number of consonants ("k", "s", "t", "n", "h", "m", "y", "r/l" , "w", "g", "z", "d", "b", "ky", "sy/sh", "ch", "ny", "py", "my", and "ry"... uh, are they all?... [No! I missed "hy", "gy", "j", "by" (06/08/10)]") Notice that we don't tell difference between r/l, which leads us to make stupid mistakes in English. Combination of one consonant and one vowel corresponds to one letter [or two letters, actually (09/11/05)] in "Hiragana" and "Katakana", which are phonetic characters. "Kanji" corresponds to one or more sequences of "Hiragana", which means a single kanji character can be pronounced in multiple different ways.
Back to the original topic, if you feel impossible to master the Japanese pronunciation, I would recommend you to pronounce like "Rio" instead. I think it would be easier for everybody and it is much closer to the real sound than "rai-oh". Well, it is not very close, but I became accustomed to the life in US and now I can recognize my name called without effort when people say like "Rio".
When I realized all these troubles about the name, I decided to give my child a name that is easy for English speakers to pronounce correctly. On October 2004, I got my first daughter and her name is "Hana". The sound is the same as the Japanese word for flower and the name is close to "Hannah", which is apparently a common name in US.
By the way, as I had been so annoyed at cafe like Starbucks and fast food restaurants when they ask my name again and again when I say "Ryo", I decided to use "Ken" as my pseudonym. I thought it would make the case much easier, and after all, the real name is not important as long as they can identify each customer. However, it turned out that I still couldn't get rid of the trouble. A couple of times, they heard it as "Dan" when I say "Ken" and I didn't recognize they called me! Well, my pronunciation is probably not good. What a pity...
Updated on 2006/08/09