Regarding the ES10 passthrough, its usefulness varies. You have to weigh the benefits of its corrective abilities against the drawbacks of its potentially-worse impact on overall image quality. The ES10 does not provide an uncolored, corrective-only pass-through signal like a traditional TBC box: the signal it "passes through" to its outputs is actually its own completely MPEG2-encoded signal, same as it would record to a DVD. Since the ES10 was not one of Panasonic's best efforts in terms of PQ, if you're very picky you may not like the combination of the kinda-lame ES10 analog>digital >analog outputs re-encoded analog>digital by your final device (recorder or PC card). Some describe the results as "nice stable mud".
The primary corrections the ES10 excels with (and other devices seem unable to cope with) generally apply only to multi-generational dub tapes or certain types of weirdly-recorded camcorder tapes. These can have two hard-to-fix issues: frame jitter, and/or severe bending or "flag waving" of vertical objects near the top third of the frame. The ES10 will correct the worst of these tapes to an amazing degree unmatched by even megabuck TBC hardware, but again the correction comes at a cost in overall PQ: your original tape gets converted by the ES10 not-so-great encoder, then goes thru another
encoding process by your final recorder system. Multi-generation digital encoding is about as desirable as multi-gen analog dubbing: not at all. If you're fortunate enough to find an ES10 with a still-functioning burner, you could record direct to that and skip the second encoding from a pass-through setup. But you'll still end up with a so-so ES10 result, and the DVDs it produced were known to have some formatting issues with audio and frame sizing (obscure issues, but they do bother some users). Its best used as a pass-through to a more modern recorder.
Like everything video, its subjective. I've seen results from using ES10 pasthrough that were really quite good, and others that made me think perhaps the original tape should just be discarded. Depends on the specific tape, your VCR, and your final recording hardware, as well as your willingness to accept various trade-offs to get a watchable digitization of rare material.
(And to second DigaDo's suggestion: anyone who needs an ES10 please PM AnubisRocks, he has one with a dead burner hes threatening to bring to a recycle center soon: grab it from him while you can!
The DMR-EH50 is too old at this point to be worth looking for second-hand. Also, be aware that Panasonics implementation of LSI was often completely different from its use by other mfrs, the typical benefits/drawbacks of LSI in similar models from other brands were not always duplicated in the Panasonics. Theres not much choice of new
DVD/HDD recorders in North America these days: if you're in Canada, its the Pioneer-derived Sony 780 (excellent), in USA its the Magnavox H2160 (also quite good). The Magnavox uses the most current, most refined version of the LSI encoder design but does NOT include the heavy DNR filters some users objected to in the classic LSI recorders. With the Magnavox "what goes in, comes out": theres no "flavoring". The Sony uses an older-generation Pioneer-exclusive encoder: it is very stable with tape input, almost as stable as an ES10, but it tends to produce somewhat muddier results than the Magnavox. The Magnavox is sharper and clearer but can suffer unpredictable glitches with tape input that you don't see until you review the recordings. Usually insignificant glitches, but something to be aware of.
If you want to split the difference, and get sharp encoding with stable tape input, the best option is one of the final Pioneer-labeled recorders which had upgraded 12-bit encoders (the Sony uses 10-bit). The Pioneer 460, 560 and 660 are superb with tape input but are now scarce unless you hit eBay. You might find "global market" NTSC/PAL Pioneers (also named 560 or 660) at importers like 220electronics or B&H. There is also the latest evolution of the DMR-EH50, known as the DMR-EH69 (or -EH65): this is another "global" model available only thru importers. Those who prefer Panasonic recorders say its as good as Panasonic gets, since I don't have direct experience with one I can't comment, but members like ChurchAVGuy have posted extensively here about the "global" Panasonics. I can say the Pioneers have worked great for me and allowed me to ditch all external processors when dubbing VHS (with a huge tape collection to digitize, the less hardware I have to manage the better).