You may have never noticed that, but it is true: All what you can save in a regular PSD, you can save in a layered TIFF-6.0 file, too. In fact – everything: All layers and layer effects, channels, transparencies, even paths, …
Is this the end of the native Photoshop Document, PSD?
Let’s take a look at advantages of TIFF: A better compression* than PSD, it is open while PSD needs a NDA and it is because of that documented and more common.
But since CS came out another file format found it’s way into Photoshop: Photoshop Big (PSB). This file format can be used with Smart Objects (double click a smart layer) within a PSD to break its file size limit of 4 GB. PSB can also be used on it’s own.
Conclusion
So if we compare all the advantages and disadvantages of PSD and TIFF-6, it turns out that TIFF-6 has not only all advantages of PSD but also advantages which a PSD hasn’t. The only reason for using that proprietary file format is, that you can extend it with PSB or use PSB itself.
Otherwise you can use TIFF without any worrying (if you don’t forget to check “Layers” in the save dialog) – I use it likewise from now on.
* multiple compressions are included in TIFF: ZIP, LZW and – without layers – JPEG