Threshold is a pretty decent scifi show. I won't say it's brilliant, because there are flaws in the storyline which I'll come to in a moment. But it does entertain and has characters you actually like.
The basic premise is that aliens are trying to re-engineer human and animal DNA into their own vision - essentially making us into them (as the show would have us believe). It's an interesting concept (although I doubt it's new to scifi) and works surprisingly well with the "enemy within" mentality. Normally I dislike shows which do this as it is usually a cop-out because they don't have the budget to do anything flash. But here it works well.
If I have one gripe about the show it's that the aliens (although apparently having access to the memories of the hosts they infect) show a singular lack of imagination when it comes to finding ways to spread the disease. The internet is the most obvious medium, yet so far it seems to have gone unused apart from one half-hearted attempt.
But apart from this, the show is good fun and entertaining. The characters are fairly diverse, although they all fit into the usual stereotypes - expert, soldier, geek, scientist etc. But at least they all have charisma, though I suspect poor old Brent Spiner is being typecast as the scientist/doctor now.
Anyway, worth a look. There are far worse shows out there.
EDIT (15.12.05) Just heard this series is cancelled. Good one CBS! Gotta love the networks' slavish attention to viewing figures. Bah!