As a former DVD 48 *user* (I still have it, tucked away) who upgraded to the BDP1, I would say yes - it is as good or better than the DVD 48 for DVD
playback, and it plays Blu Rays
Like your soon-to-arrive 7550, the BDP1 also has a USB port which you can also play mp3/wma or pictures from (You *must* insert a 1gb+ flash drive in the USB port if you want to use any BD-Live features on equipped blu-rays (plus hook up the network connection))
The menu structure/layout is also much nicer looking, and easier to use than the DVD 48.
I'm sure the analog output audio performance of the DVD48 is better, though you only need to use the BDP's HDMI connection (unless you will be using the BDP1 for a zone 2 source, in which case make an analog connection as well)
One thing I especially like about the BDP1 is that the analog composite video output is active at all times, regardless of HDMI output resolution. (It also will downmix the audio to the L/R analog output at all times as well) This is nice if you want to make use of the 7550's multiroom/zone2 video feature and will be using the BDP1 as a Zone2 source. In my home I set up distributed audio using A-Bus, and installed in-wall LCDs next to or near the volume controllers, fed from a composite video distribution amp connected to the 7550's zone2 composite video output. This allows me to see the 7550's Zone2 on screen display menu from each room, and for sources which have a composite video connection I can see the source video on the screens which makes control a breeze. With the BDP1+7550 Zone2, I can view and listen to the current source content in another room while continuing to view HD 1080p, 7.1 content in the main room. (Really enjoy this when I want to listen to concert blu-rays throughout the house)
Again, the drawback is no SACD playback. Though concert and music blu-rays are slowly increasing in popularity.
Note: If you want a blu ray player there are much better values available than the BDP1 (even at refurb prices), admittedly I paid a premium for aesthetics. I'm a sucker for matching components