Using HD software without at least an HD core card will mean you get no TDM plugins(which will be DOA when 11 rolls out anyway) so I don't see that as a big issue(was much more important with less-powerful computers from years gone by). I assume there are some other minor things you will lose, but I don't know what they are(and don't miss them). What you DO get with HD software and an ASIO interface is; all the features of the CPTK(a $2000 add-on), plus input monitoring per track(very handy for my needs) and those are well worth it(to me). I have a bit more affection for my 003 as its the controller version, and there is(to my way of thinking) a certain elegance with how well the 003 integrates(other 3rd party boxes often require some kind of routing/monitoring utility which I don't need). I do all my tracking at the 64 buffer setting and rarely even hear a mention about latency(I know which plugins to NOT use while tracking and keep ADC off until its time to mix). Sure, the 003 may not have the best sounding converters(but I defy anyone to name a song that WOULD have been a hit if only it was recorded thru netter converters) but my "money" channels bypass the 003 A>D anyway(Using a Focusrite ISA-428 w/digital card for my important 8 channels). If all you need is a few inputs, then I agree there are better choices(sonically), but if you will be bypassing with a lightpipe and/or SPDIF path, a used 003 is still a pretty solid choice(and a bargain if you shop around)
The Focusrite firewire boxes are a great 3rd party option and RME is generally considered way better(but a bit more $$).