I have to agree with all of the ladies here.
If you truly want just strength and nothing else, the PUB and PLB combo is the way to go for exactly the reasons Hollie stated. being able to complete your strength training in 2 days rather than 3 makes a bog difference, makes more options for timetabling your other days in the week.
But for the money CTX is the most value and I still do CTYX more than anything else, one whole year after buying it. I never get bored of it. Leaner legs is my fave lower body workout, and the CTX upper (get it on DVD) is, as Sherry says, phenomenal. I have gotten and still get great strength and definition gains from it and again, as with the PUB and PLB combo, my strength work is complete in 2 days, not 3. I bought the PS series, finally, earlier this year some time, and I have still to do the upper body tapes. Just can't fit it all in.
The cardios on CTX cannot be beaten. Fun, variety, challenging, innovative, intense, and as Honeybunch always says, the CTX series is still Cathe's masterpiece. For incredible versatility as a series, it still beats the I-series hands down in my view.
Clare