The issue is you get what you pay for with XBLA vs. Nintendo and PS3.
On XBLA, yes you pay $3/mo (its far less than $50/yr)
However, there is a demo for 80% of the games, trailers for most every game, free content, and all the little knick nacks for free. On Xbox, it cost.
Also, versus the other systems the online gaming aspect is better, as far as I've heard, in terms of preformance. Remember, Nintendo games have just become online compatible, and only a handful of PS3 games are online-compatible. We've had online play from Day 1, tons of XBLA games, and everything has worked.
MS is far outpacing the others. Why? They actually are investing in the system to make it work. Yes it costs, but you get what you pay for. I won't mind $3.45 a month to download demos to every game.
kn - download speeds really just depend on your connection speeds. I downloaded PGR3, a near-1GB game in just under 20 minutes on cable.