Personally, I like 'em. They're not perfect but you get good value considering the price you pay, though I do think this value statement becomes less relevant as you climb the range. My limited examples as follows:
TM5400 Positives:
Super sensitive tuner, great SD PQ where available. Never had any issues with recordings. Very good 1st timer box.
Negatives:
Crappy menus. Ugly, looks like it was built in a shed by a boffin (which it probably was!) but I prefer looking at the picture given by a box, rather than the box itself.
TM600 Positives:
Versatile (very

) in so many ways, too many to list here but trust me - it is. Good menus, has gained much sought after wife approval status. Actually looks like a piece of AV equipment - unlike the 5400.
Negatives:
Never worked consistently with my USB powered hard drive (think it needs extra power but not got round to trying it yet). Tuner/PQ not as good as the 5400 IMHO. Annoying aspect ratio problem with my box (this could well be a user issue but it's got me vexed - I can't stop 4:3 images being shown in stretchyvision).
I've also found that TM support has been above average when called upon - once by phone and once by e-mail - with the e-mail query even offering additional telephone support if required (which it wasn't as their reply was sufficient).
So, like I said, not perfect but what is?