Analysis and Caveats
The conclusions that I reach from sifting through all this data are not as clear as I thought they would be when I started this project. But conclusions are why you read this stuff, so here goes.
First, let's talk about whether there is risk in buying MIMO gear, since there is no such thing as a "MIMO" standard and the 802.11n high speed wireless standard that it is emulating is at least a year away from ratification. Since none of the current MIMO technologies are 802.11n, or can even be considered "pre-802.11n", forget any possibility of an upgrade to 802.11n compatibility when that standard is finally released.
The good news, however, is that Airgo and Atheros-based MIMO products are interoperable with existing 802.11b and g gear, although both will experience a throughput drop when non-MIMO gear is mixed into their wireless LANs. I previously examined mixed WLAN behavior for NETGEAR's RangeMax and a Linksys Airgo-based product in detail. And while I didn't verify the interoperability of any of the Atheros VLocity-based products, since Super-G is an important core VLocity technology, I don't think it's a big leap to assume that those products will handle mixed WLANs similarly to RangeMax.
But I didn't test the RaLink products for mixed WLAN behavior,so can't vouch for what happens. However, I'd expect behavior similar to the other technologies, with the only question being the extent of throughput drop.
Second, if wireless bridging and repeating or ability to attach higher-gain antennas is important, you generally will get neither from MIMO products. The one exception in this round-up is the Airlink101 router, which has its antennas attached via RP-SMA connectors (but no WDS bridging / repeating).
So as far as risk is concerned, my conclusion is that there is no more obsolescence or interoperability risk from buying "MIMO" products than any other WLAN products using non-standard throughput and range-enhancement technologies.
Next, let's consider price. More competition has lead to lower prices, with ZyXEL being the most notable exception among the products reviewed. But now that you know that you can get the same performance and features from the much more reasonably priced TRENDnet pair, perhaps we'll see ZyXEL come down in price.
Tip: Like parent Cisco, Linksys would like you to believe that their name on the box justifies higher prices for products that are essentially copies of Airgo's reference designs. But Belkin's Pre-N and G Plus are also Airgo reference clones and cost up to 30% less.
But most manufacturers are slugging it out and slashing prices with the result being that as I write this you can pick up a "MIMO" router for under $60 (the Airlink101). But given the Airlink101's inability to connect in either of my difficult test locations 4 and 5, you'll probably should expect to spend more like $80 - $90 for one of the better-ranked products.
You should note that finding a deal on MIMO client cards is more difficult, since that's where manufacturers are trying to squeeze out some margin so that they can live to fight another day. As noted previously, I suspect that NETGEAR may be doing the best at this game, given the single chip design of its WPN511 RangeMax client and its comparatively high price.