Weighted vests are awesome! All that I have seen/used have had the ability to remove/add weight at will, and they are an fantastic way of increasing intensity and core engagement during HIIT drills, and also brilliant to use during bodyweight workouts to prevent you having to go 'rep happy' to work the body (weighted chins and dips are the shizzle).
I own a TKO weighted vest, they sell it as a 40lb capacity vest, and supply the weights to match, but when I loaded it up, I and everyone I asked agreed it felt like a 'heavy' 40lbs. Put it on the scales and it is 51lbs in reality.
I went for this one out of financial limitation- it was available in the uk, and not too expensive, however
if money was not an issue, I would have had a 100lb v-max vest shipped from the states www.weightvest.com
as it was, with international shipping, they wanted 500 dollars, so no sale.
One of my best friends, and strength and conditioning coach for Cambridge University and City rugby teams has just got an 86lb xvest www.thexvest.com
and is very pleased with it. I havent had a chance to play yet, but he is finding it very well designed and comfortable.
Advantages of a proper vest over sticking weights in a rucksack are many, but most importantly, you get the weight properly distributed fore and aft, so no negative strain on your back, and the comfort of wearing, and ease of putting on/taking off inceases.
They are expensive, but if you intend to use them then they are worth it.