I recently purchased a DOA MBox 2 Mini and CPTK1 license specifically for the upgrade path to PT9 + CPTK2. It saved me about $800US. iLok licenses were transferred and re-registered to my account. It was a completely legit transaction, everything runs smoothly, it just required a little bargain hunting.

For me, the features you get with the CPTK are worth the upgrade for the higher track count, VCAs, advanced automation / editing modes, and the proposed "seamless" integration with HD systems (I have yet to personally put this feature into action). The one disappointment was that I'll have to purchase the Neyrink and X-Form plug-ins as they weren't included with the CPTK1 license that I bought. So if you go the upgrade route, be certain of what you are and aren't purchasing.

If you're simply trying to learn, I'd say save your money and just go with PT9. Learning the fundamentals & core functionality of the system should be priority one. After you have mastered that (and possibly earned a little money off your knowledge) continue down the path to expand your knowledge and toolset.

The biggest differences between 9(HD) and 9 with the CPTK are TDM processing, input monitoring, system latency, and reliable track count.

TDM processing is great for mixing, but you will obviously pay more for it. You will also pay more for plug-ins that are designed to work with the TDM chips versus what you would for RTAS/Native plug-ins. Input monitoring is useful, but not game changing (bigger in the music community than in the post/design community). System latency is system latency. It goes (from lowest to highest): TDM HD, HD Native, Native. As for track count, this is going to be dictated by your system. Even TDM HD systems have track counts that are limited by the number of TDM cards you have.

To put this in perspective, I have a session that I've run at both home (PT9+CPTK) and work (TDM HD3 system), and neither one can run the full track count. I've had plans to break it out for more detailed pre-mixes anyways, but it was an interesting test.

As far as eBay purchases go, you have to be EXTREMELY careful. Most of the HD systems that you are seeing up for auction are being resold by someone (rarely are they the original owner). The problem is this...if they aren't the original owner, they may not have all the necessary data and authorization to complete the "change of ownership" process. If you can't get that done, you're going to have a very hard time getting your system registered with Avid. If you can't get it registered with Avid, you will not be able to upgrade the software unless you end up buying yet another used license down the road...repeating any time you want to upgrade.

If you decide to purchase a used LE system for the upgrade path (as Steve did), make sure that the owner hasn't already used it themselves to upgrade to 9. There are a number of people who have upgraded using the serial number on their hardware, then sold the hardware because it was no longer required. Only 1 upgrade to 9 is allowed per hardware unit. Meaning you could be in a similar situation to what I described with the HD systems.

Summation...9 with CPTK is pretty powerful, and you're going to have to be involved in some very serious work to notive a measurable difference between 9 and 9HD...and make sure you're educated and ask the right questions before buying ANY used Pro Tools systems/licenses (be they 9 or 9HD).