I have 2 Premieres.
#1 has had a WD 1TB DVR Expander (older version) on it for about 3 years. 98% of the time it works fine, BUT sometimes when the TiVo reboots (power outage, manual, etc...) the TiVo doesn't see the DVR Expander freaks out. So far, another reboot has always fixed the issue. The danger is, if it ever does fail, you loose everything, since TiVo is clever and stores part of every recording on the stock internal HDD.
#2 (same model), I just upgraded (last weekend) to 2TB WD20EURX ($99.99 at Amazon) and it was not difficult (Thanks to everyone here at TiVo Community Forum!!!).
I agree, especially if you are out of warranty, do the upgrade, not the external, you get a more reliable solution, and DOUBLE the storage for about the same money (the WD 1TB DVR Expander is about $88 on Amazon).
Roamio Basic Lifetime (3TB HDD)
Premiere Lifetime (2TB Upgrade)
Premiere Lifetime (1TB DVR Expander)
"Nothing is idiot proof to a sufficiently motivated idiot"