Great question! There is controversy, but mainly from people marketing implants. Is it true? No, not generally. Ask yourself the question, "If you needed endo and the tooth was restorable, would you extract your own tooth, or someone in your family, and place an implant?" I doubt it! Endo works when done correctly and restored properly. If it does not work, for whatever reason, you still could retreat or do surgery and keep the ultimate implant - your own root! Implants are great, but not the only answer. Just another treament choice for the patient. Implant studies look at survivability (i.e., they are still in there no matter what). Endo studies call all teeth with signs/symptoms of disease failures. Very stringent criteria in comparison. I love having implants as an alternative, because I can't save all teeth, but I can save most of the cases I treat. A couple of abstracts that compare the two:

J Endod. 2008 May;34(5):519-29. Iqbal MK, Kim S.
One of the major issues confronting the contemporary dental clinician is the treatment decision between extracting a tooth with placement of a dental implant or preserving the natural tooth by root canal treatment. The factors that dictate the correct selection of one procedure over the other for each particular case are not yet established by randomized controlled studies. The aim of this review is to evaluate key factors allowing the clinician to make clinical decisions on the basis of the best evidence and in the patient's best interests. General considerations are discussed that will help the reader analyze clinical studies focused on this problem. Importantly, the major studies published to date indicate that there is no difference in long-term prognosis between single-tooth implants and restored root canal-treated teeth. Therefore, the decision to treat a tooth endodontically or to place a single-tooth implant should be based on other criteria such as prosthetic restorability of the tooth, quality of bone, esthetic demands, cost-benefit ratio, systematic factors, potential for adverse effects, and patient preferences. It can be concluded that endodontic treatment of teeth represents a feasible, practical, and economical way to preserve function in a vast array of cases and that dental implants serve as a good alternative in selected indications in which prognosis is poor.

Int J Oral Maxillofac Implants. 2007;22 Suppl:96-116. Iqbal MK, Kim S.
PURPOSE: The clinical question this systematic review aimed to answer was "What are the differences in outcomes of restored endodontically treated teeth compared to implant-supported restorations? Therefore the aim of this study was to use systematic review to compare the survival of compromised teeth restored with either root canal therapy followed by a crown, or placement of a single-tooth implant. MATERIALS AND METHODS: MEDLINE, EMBASE, and PubMed databases were searched for studies dealing with survival of single-tooth implants and restored endodontically treated teeth. A 2-step screening procedure was used to identify articles that met the inclusion/exclusion criteria. Fifty-five studies related to single-tooth implants and 13 studies related to restored root canal-treated teeth were included. The endpoint analyzed in these studies was the survival rate of the treated tooth or implant. RESULTS: The 95% confidence intervals for the pooled estimates for the single-tooth implants and restored endodontically treated teeth were found overlapping in forest plots for all follow-up periods. This indicated no significant differences in survival between restored root canal-treated teeth and single-tooth implants. CONCLUSIONS: The results of this systematic review indicate that the decision to treat a tooth endodontically or replace it with an implant must be based on factors other than the treatment outcomes of the procedures themselves. Both nonsurgical root canal therapy followed by an appropriate restoration and single-tooth implants are excellent treatment modalities for the treatment of compromised teeth.