Some of the down-sides have been mentioned already, but I will summarise all points including mine for a better review.
1) The odd shape (or form factor) of the unit shows that LinkSys is at an early stage of learning the art of consumer electronics design. The foot-print of the unit would actually look better if rotated 90 degrees (but you don't want to do this as the front relocates itself to the side). As a result the unit is very unlikely to fit most DVD/video cabinets due to its disproportionately long depth.
2) The intense brightness of the LED (standby/on indicator light) is an annoyance (another point of disregard by LinkSys on how consumer products should behave in the home environment).
3) The remote is very basic with cheap plastic which at times I found to be non-responsive (and frustrating indeed). TV On/Off and volume controls are provided as programmable through learning, but is that all we need to interact with a TV without reaching for TV's own remote? At least one additional programmable button must be provided to make this more useful (two or more programmable buttons are even better). With the media center extender (MCE) remote, you should also be able to switch the TV to the relevant AV input (say to HDMI input, as not all AV inputs are automatically selected) in addition to the TV On/Off, and volume controls (including the Mute function). I also noticed that on a number of occasions the programmed remote control functions (through learning) were lost for no apparent reason. In short, you cannot avoid buying a more decent programmable remote (something like the Logitech Harmony series or equivalent).
4) The delay time when the unit is switched on or off is very annoying! I can live with the booting up delay from cold, but why wait for ages to switch off (that is, from intense green light to intense red light!). If you want to be sure the unit is switched off, you have to wait for about a minute for the green LED light to turn red.
5) From 4) above follows even more annoyance with the hi-tech DVD player (up-scaling being the hi-tech stuff here). If you want to retrieve your DVD disk in haste from the disk tray (by switching on the unit), you have to wait for about a minute for it to be ejected. The cheap and cheerful DVD players that you can pick up for under £20 from food stores have a near instantaneous electromechanical response time. I see no reason why the MCE needs to complete the reboot before ejecting a DVD disk sitting idle. PCs eject the DVD tray at any point and instantly while rebooting if the eject button is pressed. Not the LinkSys MCE!
6) An update with more niggle: If power is interrupted and restored in a stand-by mode, then the unit will resort to active/powered mode rather than standby mode. Therefore, if you go on holiday, don't forget to switch off the unit (else mains interruption will set the unit to switch on!).
I also had a problem with the wireless setup (didn't seem to recognise the security passphrase) and being tired of everything wireless I opted for the wired connection which fortunately fitted with an ongoing DIY for putting the RJ45 cabling in place. No problems at all with the wired network but suspect others may have an issue with the wireless setup particularly if security is enabled (which is highly recommended anyway).
OK, a lot of points on the down side. On the positive side? I think the MCE provides a multimedia experience close its multimedia PC server and I am impressed by the quality of its main function (i.e., extending media centre to the living room). It is a shame that the rather aesthetics issues listed above (important to the consumer electronics market) are somewhat overlooked in the design of this otherwise well performing equipment (hence the four star rating!). It is possible that some of the criticisms can be addressed through firmware update (such as the slow response of disk eject) and I look forward to LinkSys's positive response in this regard. The excellent performance of the MCE has persuaded me to live with the annoyances (namely, the unit protruding off the back of the storage cabinet, having to use another remote, etc). Good show Linksys [in the IT and engineering department] BUT give due consideration to the art of consumer electronics [aesthetics, ergonomics, and the like].
Finally, a feature request to make this MCE even better: please please incorporate LAN-Wake-Up of the PC through the remote control so that the media center PC can be turned ON remotely (meaning that you don't have to walk to the PC to switch it on). The LAN wake up technology has been around for a long time with Ethernet network cards so this is a doddle for LinkSys to throw in. Yes, you can do it through infra-red remote extension, but wouldn't it be nice if all is done through the MCE? This also has an 'energy saving' slant (i.e, switching on PC only when needed) which nowadays seems to be one of the talking [and marketing] points.