I have an ergo - not a beco - but they're similar and I've heard great things about the beco. I use my ergo on my 19-month old son who is 24 lbs. I've used it on my 16 month old nephew who is 32 lbs, too. Both ride comfortably.
Technically the weight limit on the ergo is 40 lbs (same as beco I think?) - I love it because my son at this point is very busy - sometimes he wants to ride but lots of times he wants to get out and walk. This is so easy to get on/off that we can do that with no problem. (When he was very small we mostly used a wrap-type sling, but that required tying).
The limiting factor for this carrier for me is that I am now 4 months pregnant and I can tell it's going to get uncomfortable to have anything around my waist soon. But I don't really think there are *any* baby carriers that avoid that problem really.
When I was looking at carriers I read a lot of reviews at www.thebabywearer.com - very helpful website.