IMHO and for my ears, ㄹ definitely sound different, like R or L, depending of its place in the syllable. Although, if you ask a native Korean speaker, he or she would tell you tell you that there is no difference, probably because native speakers aren't always aware of some aspects which comes naturally to them.
So the general rule is that when ㄹ is pronounced L only when it's the last consonant of the syllable and the first letter of the following syllable(if any) is not a vowel or ㅎ. Here are some examples:
1) 사랑 (saRam)
in these cases ㄹ is the first letter of the second syllable and sounds like R
2) 날씨 (naLshi)
sounds like L because ㄹ it's the last letter of the syllable and the next syllable doesn't start with a vowel
3) 말아요 = Don't (maRayo)
일해요 = Work (iReyo) - in this case ㅎ becomes silent
sounds like R because the next syllable starts with a vowel/h sound
4) 실례합니다 = Excuse me (shiLe hamnida)
알렉스 = aLex
아렉스 = aRex
In this case I noticed that in order to obtain an L sound at the beginning of a syllable, you have to double the ㄹ. For example, if I'd wrote my name 아렉스 instead of 알렉스, it would sound aRex instead of aLex.
I hope it makes sense what I wrote above :) I guess the above observations also depends on the perception of the R/L sounds in your native language. For my native language, they make sense.